Mega Tyranitar and Mega Gardevoir are the two I like more when it comes to use them in battle. Tyranitar if invested wisely in defense can at least survive a hit from anything not setted up and attack back with an always powerful hit, while Gardevoir with the right support here and there can sweep with a pixilate hyper voice that no substituted foe can survive...unless specially defensive....or steel type.
For looks, Mega Beedrill, Mega Audino and Mega Mawile. First's a friggin insect drone, almost robotic (in fact a bug/steel typing wouldn't bee too farfetched for this pokemon, just look at all those metal spears!) while the second's a beautiful white mage that provides with a "Gandalf the white+Madoka Magica" kind of essence to the battlefield, I love it. The third is the queen of piranha plants, and everyone knows what M-Mawile does with that attack stat, lol.
Now that I'm at it, I should also point out my least fav megas as well, because I'm too lazy to see if there's a "least favourite mega" thread. So here we have Mega Slowbro (NEVER let it set up or it won't die, rest eliminates whatever burn or toxic stall you might be planing, while calm mind makes it impossible to kill by special means plus ice beam is a very hard-hitting move). It's design isn't really bugging me: it's silly but understandable assuming the progress of the soul-consuming shellder: today your tail, tomorrow your whole body.
And my second least favourite, putting aside the overpowered beasts such as M-Metagross and M-Salamence (that I don't like but I can understand why others do), is M-Rayquaza. Mega Mewtwo is almost there, but not quite because I like the guys (even though I would have prefered only ONE mega for every mon, and also how the hell does mewtwo have one, what bond do you have with any trainer you depressed embrio drama queen?). Regardless, M-Ray quay quay is disgusting. More powerful than god, more powerful than mewtwo, what are you doing ray quay quay? you can't do that unless you're in dragon ball, but you're in pokemon, don't overpower everything...