Tony Stark like totally wanted to kill Steve for lying about the parents thing, but I'm pretty sure he would have started with the Winter Soldier. Plus he and Hulk are science bros, right? Oh! And he's a kid right now, so I doubt he'd get the puberty arc plus a revenge arc. Makes no sense.
I still say Wanda, because she's always conveniently crazy in the comics whenever they need somebody to do lots of damage. What if she's going NO MORE UNIVERSES or whatever? Also come on, do you totally think all her retcons about her parents are her like magically... You're not my real dad, Magneto!!! Or do you think it's a Disney vs Fox issue?
Umm... Oh. That bald Shield dude that was actually Hydra all along? I don't remember his name. I bet it's him.