Mostly, I like how varied the designs are.
You want tough? Have Garchomp, Tyranitar, Aggron, Nidoking, Machamp, Gyarados, Steelix, Sharpedo, Groudon, Zekrom, most fully evolved starters...
You want cute? Have Raichu, Whimsicott, Clefable, Wigglytuff, Blissey, Lilligant, Cinccino, Furret, even Dragonite and Goodra...
You want elegant? Have Gardevoir, Suicune, Articuno, Milotic, Lilligant, Aurorus, Diancie, Absol, Altaria...
You want weird? Have Slowbro, especially Mega Slowbro, Reuniclus, Mawile, Forretress, Stunfisk, Shedinja, Masquerain, Claydol, the Regis, Klefki, Malamar, Zygarde...
You want ugly? Have Weezing, Garbodor, Bastiodon, Purugly, Skuntank, Seismitoad, Diggersby, Barbaracle...
This also extends to the various inspirations: real animals or plants including pretty some obscure ones, but also a bunch of weird Japanese folklore, and even the object-based Pokémon some people utterly despise (I generally don't, plus at the very least Bronzong is pretty high in my favourites list). Pretty much every design, even the weirder or uglier one, is bound to be liked or even loved by someone, somewhere.
And belonging to a category doesn't preclude competitive viability, as some of the cute ones are every bit as viable as the toughest-looking ones.
I also like how it feels like most Mega Evolutions were designed "differently" from normal evolutions. Normal evolution is a natural process, while Mega Evolution is a temporary power boost given to an already "adult" form: therefore, it is completely sensible that in most cases Mega Evolutions look like just a fancier form of the base Pokémon instead of something completely new. Compare, for example, the difference between Electrike and Manectric and the one between Manectric and Mega Manectric, the same goes for Gengar, Tyranitar, Medicham, Camerupt, Ampharos... no more growth, just power.