At this point I'd say BA. His mobility is amazing and as a Joker main I spend almost 3/4 of the match just trying to get him open. Superman is a close second but most of them are so predictable now, so I can get in way easier. With BA, I have a hard time reacting so I either get caught by the lightning or the divekick.
But mark my words, as soon as this patch comes out it's gonna be MMH and Bane (yes, Bane) competing for top dog. MMH, like a lot of you have said, is just so versatile and unexplored that he's gonna start kicking ass as soon as tech is developed for him.
Bane, on the other hand, gets damage reduction during his Venom boosts on top of his damage boosts, which means if you space out venom well there is no way the cast can go toe to toe with him, and if the Bane player plays a good defense you won't be able to get a full combo on them. It's scary to think about, but I guess we'll see what happens.