Sceptile, Aggron, Mawile, Zard X, Lucario

Mega Gardevoir

I would have preferred that the Pokémon simply received an additional evolution, even if that meant giving some a fourth stage, rather than making it a temporary state for which you have to sit through the animation every battle.

Yes, limiting you to one mega evolution and denying the Pokémon a held item does help balance the mechanic a little, but we could also just balance the new evolutions properly in the first place.

That aside, I like it when mega evolutions give Pokémon a new direction in their design, rather than being the same Pokémon but more and edgier.

I really like both Mega Mewtwos for that reason. X is particularly appealing to me given that fighting type is not an immediately obvious second type for what has arguably been the most powerful psychic Pokémon. Y visually does significantly deviate but it looks kind of goofy to my eye.

I also like Mega Charizard X for finally shedding that 4x weakness to the sneaky pebbles, even if the colour scheme does somewhat eat shiny Charizard's lunch.

My favorite mega evolution is Mega Tyranitar, but I also love Steven's Mega Metagross from the anime!