There are many things we've seen in MLP, so I want to classify social roles there.
There are basically four races in Equestria. If we're using the same classification as social classes, then it'll be:
Underclass - Earth ponies
Low class - Pegasi
Middle-Class - Alicorns
Upper Class - Unicorns
Now the explanation:
1)I guess everyone universally agrees that earth ponies are inferior to all three races. There's nothing they can do that the other can't, and there are tons of things earth ponies can't do that the others can. It's the difference between a cripple (earth ponies) and a healthy person (other races). They can have strengh, sure, but it's mostly from training and stuff like that. Rainbow easily beat Applejack in hoofwrestling, and the only reason why AJ was able to kick harder is because of years of applebucking (as she confirmed it herself). If RD was kicking apples the same amount of times, she'd have been able to do the same, AND able to fly.
As for growing plants, Sunset Shimmer was able to grow plants herself without any help from earth ponies. (About comics being canon: https://derpicdn.net/img/view/2016/4/3/1123849__safe_comic_text_meta_idw_twitter_slowpoke_jim+miller_canon.png
2)Pegasi can fly and control the weather. That alone makes them superior to earth ponies, but they are still vastly inferior to alicorns and unicorns.
3)Alicorns can raise Sun and Moon, and basically have the same stats, as unicorns and pegasi. But as you know, it doesn't make them superior to the others, and I'll explain why.
4)Unicorns. They are truly master race, because they can do anything they want. Flying? No problem. Weather control? Some practice and it'll be easy as can be (Magical Mystery Cure). Plant growing? You got it (Sunset Shimmer). Super Strengh? Twilight was able to lift Ursa Minor, while doing a lot of other things with her magic. Sounds much more impressive, than reducing a rock with hooves, don't you think?
Plus, Unicorns are superior to Alicorns, as well. They are able to raise Sun and Moon, as well, but also they are stronger in magic. They can self-levitate, and they are able to easily overpower alicorns. Starlight Glimmer tossed Twilight (one of the most powerful alicorns and the element of magic) easily, without even straining herself. She was able to create an unlimited time travel spell, which makes her even more powerful than Discord (he had some limitations with time travel). Her shield was invulnerable for any Twilight's attacks.
So, one unicorn can easily overpower an alicorn. And what if said unicorn will study magic? What if ALL unicorns will study magic? Nobody said that unicorns can't study magic, unrelated to their talents. It'ld be just harder for them, but still, they are able to do this.
Let's say, five unicorns: Starlight, Rarity, Sunset Shimmer, Lyra, Moondancer. They can easily overpower five alicorns (as we've seen in Season 5 finale). And there are THOUSANDS of unicorns. Do the math.
So, it raises the question -- why they don't do it? It's simple -- because they don't want to. Have you noticed that they don't have any magor responsibility as a race? Earth ponies have to grow food, Pegasi -- make the weather, Alicorns -- raise the celestial bodies. Yeah, they used to raise Sun and Moon before, but since alicorns appeared, they don't have to.
They are just allowing the other races do all hard work for them. Why would you need to raise food, when earth ponies will do it for you? Why do you need to cause rain, when pegasi do it for you?
Before you ask, they are allowing alicorns to rule, because they hate all the paperwork. As we've seen in comics, Luna's gone mad when she tried to take Celestia's work on herself. Why would a unicorn bother with it? They are content with a fact, that they can easily overthrow alicorns, any time they want.
So, unicorns are truly master race, as we've seen from latest season and comics.
Any thoughts?
It's all wonderful until you remember Troubleshoes, who was the only pony, who ended up in a jail. And then he had to face the charges for damage he accidentaly caused. Except for the last time, when CMC did it for him. Forgiveness for some, but not for all -- that's what the show implies. I don't remember Trixie facing charges for the damage she caused, even if under the influence of the Amulet (Troubleshoes didn't mean it, as well). I don't remember Dash facing any charges. So, we either forgive everyone, or punish everyone. The current situation with Troubleshoes is called double standarts.