MegaMawile and MegaScizor are, without a doubt, my favorite Megaevos. I've tried out Megagar in-game but it doesn't seem to be too exciting (I prefer regular Gengar, at least that way I could let it hold recovery items such as Black Sludge). I've also tried out Mega Lucario to a certain degree of success and I think it's really cool, but those former two stand out the absolute most for me.
I really want to include Mega Venu as my favorite also, but I've never tried it out myself to really judge anything on it. I've seen it used before and it survived a rather powerful ice beam, which I think is really cool. n_n
Houndoom, Kangaskhan, and both Charizard Megas are my favorite Mega Evolutions thus far, and I'm slowly warming up to Ampharos thanks to that fabulous hair. I'd love to see a Mega Dunsparce or Serperior, ideally with the first two being part Dragon-type. Mega Zangoose, Seviper, and Sableye would be pretty cool, too, as well as... oh, screw it, there are few Pokemon who I wouldn't like to see a Mega Evolution for.
Basing all this solely on design, mind you. The only Mega-Evolution I've used is Mega Blastoise and it's... okay, I guess. Not much of a step up from normal Blastoise from what I can tell, although I'm not too big on stats or competitive battling or the like so I wouldn't know. :V
I like Mega Absol, Charizard X, Mewtwo Y, and Gengar. I do theorize they'll give every starter a Mega (it's weird to give Blaziken a Mega if none of the other Hoenn starters do, for example) and I can't wait to see Mega Meganium and Sceptile :0
I love mega Absol and mega Ampharos, they have great designs <3
the pokémons that I would love to see getting a mega evolution is
Rapidash (really wish that if they give it a mega evolution, that they will give it a Pegasus look)
Arcanine (is a great pokémon, and personally, they shouldn't do to much change with it, since it's such a beautiful pokémon already)
From the ones I've played with Mega Venusaur looks to be my favorite, especially considering this was my first pokemon in the series. Still, I really wanna like Mega Mawile, if only its priority move didn't rely on chance.
As far as Mega I wanna see, Infernape would be perfect for when Pokemon Z rolls around. It could have an orange vest and glow red when making physical attacks. A Kameha.... Warcraft Troll casting stance (™) for the special attacks.
I think my favorite Megas are Alakazam and Medicham, because I think they have the most thought put into their designs. Megakazam's beard and yogic lotus pose make it look like a true sage, and Megacham's new form looks like a regal and ancient Hindu deity, especially because of the "arms".
I like Mega Absol too, though it took me a while to grow on me. It looks like a big, fluffy sphynx...
I guess I like the ones that remind me of mythology and folklore?
I'd love a Mega for Milotic, just to make it even with Gyarados. Water/Faerie, maybe? Roserade would be a nice Mega to have, too! I could see it keeping its typing.
Megas i really like: Gengar, Banette, Houndoom, Mawile, Gardevoir & Mewtwo Y. Prefer Mewtwo Y rather than X even with the bottle-opener head.
I like how Mega Alakazam looks like a guru & mega Medicham looks like a yogi; some very nice design work there.
I don't understand how Mega Absol looks the way it does yet isn't part fairy-type?? Dark/evil faeries exist in mythology so why not?
Pokes that i'd like to see: Sableye (definitely needs a boost and would look really demony), Nidoking & Nidoqueen (ever since i heard about the fake Nidogod & Nidogoddess), Farfetch'd (could use a stat boost and an extra leek), Jynx (just cos she's cool and could look like an ice-queen), Porygon-Z (could be interesting to add even MORE technological advancement), Wobbuffet (hey, Wynaut? lol), Girafarig (the tail-end could become the dominant side), Unown (to form the words listed in the unown report and with more joined together they'd have more power ergo be less useless. Maybe gain Psychic attack & change ability to Telepathy), Shedinja (with decent HP it would still have WonderGuard and therefor would still be able to be hit), Manaphy/Phione (could be super-cute), the lake trio (i find their appearance & backstory boring, they could be expanded upon to make them less lacklustre)... just some ideas.