My least favorite would probably be Mega Metagross - I just don't like its design, compared with regular Metagross, and...the whole thing where it fuses with an additional Metang and two Beldums to Mega-Evolve is hard for me to swallow. Like, I can suspend my disbelief about Beldum and Metang fusing with other members of their species to evolve even when they're the only one in your party, but the idea of random Beldum and Metang coming out of nowhere every single time you Mega-Evolve and then going on their merry way once you're done is just too much. :/
Close behind is Mega Heracross - I'm not a fan of regular Heracross (again, design just doesn't appeal to me), and the Mega really didn't improve on that.
Some of my favorites...
Mega Sceptile (I remember when ORAS was first announced, I had no idea which starter I wanted to pick - then I saw Mega Sceptile, and the decision was made)
Mega Gardevoir (especially when it's shiny)
Mega Charizard X (I know, I know, 2edgy, but I legitimately think it's gorgeous - that blue-and-black color scheme, the blue flames, the ragged wings...😍 I'd wear an outfit inspired by this design.)
...Mega Kangaskhan is my guilty pleasure Mega. u_u I know it's the scourge of the metagame, but the concept is adorable and I want to cackle evilly whenever I get one in the Battle Factory, so there!
Also, shout-out to Mega Gengar and Mega Garchomp for tackling the "shiny Gengar/Garchomp" problem.