Like almost all of the seasons, it's rather inconsistent. After three really good episodes in a row, the season got stuck in mediocrity with only Make New Friends and Slice of Life helping out the quality. But there's also been some hope, as Amending Fences is the best episode so far this season.

My list from favorite to least:

1. Amending Fences (all-time favorite)
2. The Cutie Map
3. Slice of Life
4. Castle Sweet Castle
5. Make New Friends, but Keep Discord
6. Bloom & Gloom
7. Party Pooped
8. Do Princesses Dream of Magic Sheep?
9. Appleoosa's Most Wanted
10. The Lost Treasure of Griffonstone
11. Tanks for the Memories
12. Princess Spike (the only one I hate)
    

But if I were to grade them based on actual quality, the order would be as follows:

1. Amending Fences: A+
    
   It's a type of episode children need desperately. What if one decision that looks inconsequential can be a terrible mistake with terrible consequences. Twilight's decision to skip MD's party only forced her to become reclusive. Moon Dancer had many great reasons to behave the way she did, and she bluntly called out Twilight. The moral is very mature and one Twilight had to learn. Not just the best episode of the season so far. It overtakes Sisterhooves Social as the second-best of the show and easily competes with The Best Night Ever.
2. The Cutie Map: A+
    
   I rarely give out A-pluses. Season four is the first with two. The Cutie Map is the first, and was it REALLY written well. Starlight is the best villain of the show. The M6 are written great. The town is creepy, yet wonderful. The allegories to communist societies and cults are accurate and disturbing, including the use of genuine brainwashing techniques into FIM. This episode mandates analysis, because there are so many layers here. Easily the best two-parter of the show.
3. Bloom & Gloom: A-
    
   It may be one on the boring side, but there's a purpose to the structure. Apple Bloom's worries over what type of cutie mark she'll get make plenty of sense. Not simply because she's a child. But also she still doesn't know her place in the Equestrian world. The one big downside is Babs got her cutie mark via exposition, and the episode never relented the idea that she's no longer a CMC now that she has one. Just because she has a mark doesn't mean her role as CMC is officially over.
4. Slice of Life: B+
    
   To get the flaws out of the way. The story structure is rather weak because part of the story revolved around the bugbear in the background. Lyra's and Bon Bon's voices are stereotypical valley girls. Some of the reference humor doesn't work, like Bon Bon's secret agent spiel and the Twilicane. But there's a lot of nice characterization, especially Derpy, Dr. Whooves, Vinyl, Octavia, and Steven Magnet. Gummy's philosophy on life is absolutely hilarious. Luna and Celestia talk to each other like actual sisters. Many of the references blend in the canon, especially the Vinyl and Octavia duet. The sound moral fits both the story and relationship between the bronies and DHX. It's a lot better of a story than the detractors give credit for.
5. Castle Sweet Castle: B
    
   Spike was nicely done here; despite being comic relief, it was brief, and he caught a break. Twilight's grief over the Golden Oak Library by staying away makes so much much sense for her character, and it addresses its empty, cold feeling. The roots of the Library above the roundtable is a very fitting tribute. Although her friends acted stupid, it made sense because they wanted to make the castle feel like home for her, and they figured out the problem themselves. One problem is the pacing: the middle goes by really slowly.
6. Make New Friends, but Keep Discord: B- (may drop to C+ or C in the future)
    
   Discord was great. Fluttershy was great (at the end). The moral is one everyone understands, works with the context. Some of the more obscure pop culture references work really well with Discord and the narrative. But the best part was Maud's "You're the most basic of jokes." But it's also bogged down by very blatant pop culture references (which risk making the episode look corny in the future), using a clichéd jealousy story (the same structure as Owl's Well, although done better), placing no blame on Fluttershy for perpetuating the arc and ignoring him, and Tree Hugger being the stereotypical stoner hippie.
7. The Lost Treasure of Griffonstone: C
    
   One of the episode's biggest strengths is its portrayal of Gilda. After being presented as a 90's stereotype, her vocabulary was toned down, yet she still retained that edge. Rainbow Dash and Pinkie Pie were also really nicely done, though Dash stereotyped. The problems, though, are severalfold.
    
   One: The entire griffon population was portrayed as greedy, when only one was (and his reaction was understandable). Griffonstone is apathetic, in complete poverty, and agriculturally bankrupt.
    
   Two: Twilight acted like a slave to the map; this is one such episode where a throwaway line or two or letter would've worked.
    
   Three: the episode paints friendship as the be-all end-all. Friendship can help rebuild your own community, but it can't repair a bankrupt government. In short, the worst moral this season.
8. Do Princesses Dream of Magic Sheep?: C- (may upgrade to C in the future)
    
   Mature morals are very difficult to tackle while staying true to a general-audience content rating. Therefore, sometimes allegories are the most appropriate way to tackle dark themes. This episode has the guts to tackle self-harm. Princess Luna was able to be forgiven by her sister and the rest of society, but she never forgave herself. The Tantabus was a great way to portray physical punishment to repress her psychological torment. But sometimes it gets so addicting, you want more until it gets out of control and threaten lives.
    
   But there are several problems.
    
   One: the fan pandering in the middle really clogs up the story. Cut all of that out, and you can really expand the depth of Luna's self-punishment.
    
   Secondly, there's way too much exposition; no one needs to hear "Tantabus" and how much of a threat it is all the time.
    
   Lastly, the pacing is absolutely terrible. The middle is played a lot while the beginning and ending are really cut short. Secondly, do you really think Princess Luna will be able to immediately forgive herself and mentally heal in just one minute? No. People who have these problems take a really long time to completely heal; sometimes they don't at all.
    
   Self-harm is a really delicate subject, and it wasn't treated with any respect. Shameful.
9. Party Pooped: C-
    
   There are episodes that play to specific character's strengths. Party Pooped played to two specifically to Pinkie: zany cartoon logic and her determination. She willed to make the party 100% authentic to Yakyakistan's culture by going up to their country…only to fail. The Mane Six deserve a lot of credit for keeping their composure during the yaks' visit, especially Twilight. TS's mini-freakout makes plenty of sense.
    
   But there are a few problems.
    
   One: The show remains inconsistent with Equestria's geography. It makes no sense for anyone to travel from Equestria to beyond The Crystal Empire in a short amount of time; at most, it'll take at least a day.
    
   Secondly, the yaks are one-dimensional stereotypes of primitive people. Based on DHX'ss location, you can assume the yaks are Native stereotypes thanks to their broken language, terrible temper, and warrior-first ideology. They can also be viking stereotypes for the same reasons plus their lack of knowledge of Equestrian culture.
    
   Thirdly, the yaks are portrayed as the antagonists when the Mane Six had no idea how to truly celebrate the yaks' culture. The yaks were promised of complete authenticity and felt lied to, hence their anger. However, they still had zero right to vandalize and get away with it.
10. Tanks for the Memories: D+
     
    The first S5 episode I outwardly dislike. Granted, DHX has the guts to create an allegory of death, and the character makes sense. Rainbow Dash loves company; once Tank brumates (the reptilian version of hibernation), she'll be alone for several months. But the allegory is completely inappropriate because the stakes aren't high enough, but the audience is supposed to believe it is. Secondly, Rainbow Dash is very out of character; she understands how important hibernation is to not just the animals, but all of Ponyville/Equestria. She's the weather team captain. But what really hurts her portrayal the most is the fact that she understood the terrible consequences of her actions, yet doesn't care as long as Tank is still with her. What's worse is she never got called out and never got punished for vandalizing the Weather Factory. The moral of being okay to grieve is great, but it was bogged down by some terrible characterization of Dash and huge implications of her actions.
11. Appleoosa's Most Wanted: F
     
    This episode showed how much poor comedy can really damage the integrity of an episode. Even after the CMCs were warned to not wander out into the forest alone out of fear that they'll be killed, they still ventured out in hope of finding their cutie marks. This is The Show Stoppers's version of the CMCs, which they grew from. But the biggest problems stem from the comedy and Trouble Shoes himself.
     
    One: TS is the constant victim of slapstick, and he resides himself to a fate of clumsiness and bad luck. But slapstick is supposed to make the audience laugh; slapstick is supposed to be used for characters who deserve comedic karma, but he doesn't deserve it. Come the climax, the audience is supposed to laugh at the jokes, but TS doesn't see it as funny; he just wants to be a part of the group at the rodeo, not be a clown. It's a major asspull for him to suddenly be okay in being a clown.
     
    Two: Trouble Shoes's clumsiness was initially written to be very dangerous. When he knocked down the hay pyramid, everyone panicked. But then later in the episode, the audience is supposed to not see it as a danger and laugh from it? Stay consistent to your own episode continuity. If we were supposed to laugh in the climax, then the beginning shouldn't have treated this episode as a danger.
12. Princess Spike: F
     
    The worst episode of season five so far, and the second-worst written episode of the show. All of the citizens in Canterlot was a groveling suck-up to Twilight, while Spike was being ignored just because he's not a princess. The moral is supposed to be how he took advantage of others to make himself feel good, but it's a non-sequitur; the base of the conflict occurs when he takes Twilight's name out of good intentions and hopes that Twilight can move on with organizing the convention without all that backlog. Spike has good intentions, but his character development from past episodes is completely reset here. I really hate Princess Spike.
    

What went right?

2. The characterizations of many of the Mane Six are a hell of a lot better this season than last. Twilight's character, which was heavily reduced and flanderized for the most part, was brought back in ways that make sense: her adorkableness, intelligence, and figuring out problems without using a DEM. Pinkie Pie's characterization is completely on point with a balance of humor, seriousness, and Pinkie cartoon logic. Minus her portrayal in most of Make Friends, Fluttershy's behavior has significantly improved by being less shy and sticking with the lessons she learned.
3. The morals are greyer, and most of the themes are very mature, all of which I highlighted here and here. For a show designed to be revolutionary, this is incredibly important. If going by the titles and animatics, we'll likely see more in the second half.
4. Animation has improved via new facial expressions, lighting, etc.
5. Mostly great background and secondary characters. Several are memorable like Double Diamond, Sugar Belle, and Moondancer.
6. Starlight Glimmer. Best villain of the show so far.
7. Some really good songs, especially In Our Town.
   

What went wrong?

1. Spike's characterization in his prominent episode is an absolute mess. To make it worse, the ending doesn't hide how much of a joke the staff treats him. This is a pro-feminist show; the treatment of his character questions the integrity of the whole show. If you can't write him correctly in his episodes, keep him as a secondary character.
2. Once again, it's still very inconsistent. To make it worse, many episodes that should've been good — Do Princesses Dream…? — are lost because of bad writing choices.
3. Way too many stereotypes. Tree Hugger (stoner hippie) and the yaks (primitive citizens) are lazily developed characters. As funny as the Barney parody was, it's also a stereotype of masculinity by flanderizing Dash's tomboyish portion of her character. Lyra and Bon Bon have stereotypical valley girl accents.
4. Rainbow Dash's characterization is hit and miss, sometimes relying on the stereotype mentioned above or disregarding continuity to force the plot forward.
5. Some episodes are poorly paced. Castle Sweet Castle and Dream poorly flow from one area to the next.
   

In short, it did really well, but it can also do better.

Source: What are your thoughts on the first half of Season 5 so far?