I would have said none of the characters, but now that people mention X-23, gameplay-wise, I think I would get rid of her. She was never really that fleshed out or unique aside from some cool supers (particularly her Level 3).
Taskmaster always even seemed hobbled together a bit, and I even mained him for a while.
Akuma is a staple and all, and again, I mained him too, but if someone had to go, I would just integrate his moveset into Ryu and not much would be lost, especially after Ryu could combo out of that overhead in UMvC3.
So yeah, on the basis of gameplay contributions, I wouldn't be too upset at losing the following, especially if you get much more unique characters in their place:
X-23
Taskmaster
Akuma
Taskmaster has to make it lol! my favorite character out the bunch. I can do away with Black Panther getting something similar to X-23 moves. Akuma being a ryu special like MVC 1 is ok. Zero has to go again its Megaman Time to shine. Iron Fist i can see go and Daredevil replacing him.