Some megas may have better stats and abilities than others, but there's no such thing as the best mega evolution. It all depends on how you use it, really—if you have a good strategy, then you could easily win a match even with the weakest mega Pokémon (stats-wise). d:
In terms of competitive battling, Blastoise's mega form has gotta be one of my favorites. Its defensive stats are heavy, allowing it to take quite a lot of hits and stall your opponent for many, many turns. I've once battled one of my friends with Mega Blastoise. During the battle, her Raichu managed to use three Thunderbolt attacks in a row, which Blastoise was super-effective to—it still didn't faint. Not only is this Pokémon a great staller, it makes a great special sweeper, too. Its Special Attack stat is pretty high, and thanks to its Mega Launcher ability, it gets an additional boost to most of its special-based attacks. Water Pulse, Hydro Pump, and Flash Cannon would be some great examples. Well, I'm not exactly sure about Hydro Pump and Flash Cannon, but Mega Launcher does affect Water Pulse. I've tested it myself a while back.
I'm also quite a big fan of Mega Charizard Y. It may not be as powerful as Charizard X offensively, but it does have a gargantuan amount of Speed. This makes it a great revenge killer. :) I've used this Pokémon a few times when battling online. With the help of it, I've been able to take down a lot of my opponent's Pokémon without even breaking a sweat.