Aww. do I have to chose? I like almost all of these a lot.
Wonderbolts Academy - A perfect Rainbow Dash showcase that finds just the right balance between her charisma, her values, and her insecurities. She clearly takes her role very seriously, but her principles come first - and the episode is smart enough to have her slowly realize that rather than immediately take a stand.
Horse Play - Honestly, this is tied for first place to me. I love seeing Celestia given new personality traits, and even more than that, this is just a terrific late example of what this show used to be good at. I love how, even though Twilight does the wrong thing, she does so for a perfectly sympathetic reason. Plus, everyone else is hilarious and perfectly characterized here.
May the Best Pet Win! - On the other hand, this one is a bit more simple, but it's also just irresistibly silly. Dash providing her own music stings for a fast-paced animal race is just an inherently fun idea, and it's great to see her showboating just because she feels like it and not because she's overcompensating. And then it's got a great moral about the value of persistence on top of everything else.
Make New Friends but Keep Discord - In general I just like Discord episodes, and this one is great because his rambling goofiness is essentially let loose, with Fluttershy (and Maud) only occasionally countering it. Tree Hugger doesn't counter it at all - if anything, her own goofiness just heightens it. Funny stuff.
Parental Glideance - I still have issues with the moral here, but I just find Rainbow's parents delightful. They're embarrassing, and yet they're also so earnest and well-meaning, and nobody but Rainbow actually seems bothered by them. Plus, Rainbow and Scootaloo are both characterized perfectly here.
Dragonshy - This is one of the more confident episodes of early season 1, but like a lot of the first half of that season, I mostly just find it cute and funny. Fluttershy just getting mad and talking to the dragon is great, as is her various attempts to just go home. It still has the lightweight, non-serious vibe of early season 1, but it also seems like the characters had stabilized by this point.
Newbie Dash - The impressions sequence basically means I never want to watch this again, and maybe it should have had an anti-bullying message in there somewhere, but I don't see this as an episode about Rainbow being humiliated. I see it as a story about how far she has come, and how scared she is of losing all she's gained. There's some powerful stuff in there.
She's All Yak - I really do love Yona, and she's adorable in this, but I really don't want to see the mane six doing racist things. This takes on a whole lot of baggage for a story that doesn't engage with that baggage beyond "be yourself," and I find it a bit depressing. And besides, I preferred when the mane six were trying to fight racism.
Bats! - I feel like there's a lack of balance here. Sure, fruit bats are an essential part of the ecosystem, and I think everyone would prefer to treat wildlife humanely, but Applejack's livelihood depends on those bats, and the episode never gives her side a fair shake. And then the Flutterbat stuff comes out of nowhere and feels like a distraction.