I thought Mawile already looked cute, but the Japanese style "clothing" plus the twintails are adorable.
Didn't care much for Lopunny before, but the added Fighting type is clear through the tattered clothing. She also becomes less shy after Mega Evolving.
Camerupt looks like a giant fluffball that had a bowl of soup put on top of it. And it's Team Magma-branded!
I like Absol's Mega Evolution because it showcases a different side of the Pokemon. Before, it was just a bringer of doom, but now it's more of a savior.
And Mega Rayquaza just looks plain awesome.
The idea behind Tyranitar, Aggron and Houndoom was just "ADD SPIKES!!!", Manectric looks like a lightning bolt with legs, Heracross changed species somehow, and I just don't like how Latias and Latios look the same apart from the eye color.
Mega Latios & Latias need tweaks with the design, they look identical with the exception of the eye color.
I would like it if they just kept the cuteness that Latias had, but..meh..
Mega Manetric...poor thing..I really liked you at one point. :C
Other than those, favorites are Mega Charizard X (Sorry, but...it's great), Mega Heracross, Mega Mewtwo X, and the god-like Mega Rayquaza (I haven't even played ORAS yet, but what I've heard..it's unstopable)
I think the best is Mega Mawile.
Mawile is my favorite Pokemon, so upon seeing it's Mega Evolution, I flipped. It's also crazy powerful. (HUGE POWER + STAB BOOSTED PLAY ROUGH = CARNAGE)
My least favorite, I haven't really used at all, but I don't like Mega Lati@s. Mainly because they're exactly the same. If they looked different so I could make out the two clearly, I probably would appreciate them a lot more.
I don't like the Mega Latios and Mega Latias overall design. You can't even distinguish them. It would've made a world of difference if they retained Red and Blue. Shiny form makes them even worse.
I really like Manectric's mega lol.
Same with Lati@s. Not that I "like" them, but there more of a... storyline related mega? than a battle-ready mega. I mean we can soar on them so that basically makes up for their not too great designs (I do kinda like their designs tho, but their shiny colors suck).
that aside, my favorite's would probably be Steelix, Slowbro, and Banette, even though I haven't used any of them.
Don't really have any least favorites, since I really like how unique they all look, but if I had to pick my "least favorite" I'd pick mega Beedril, because every time I look at it, all I can think of is this;