Any, at all, and you're still you when you chose, you just have this heroes' powers
I definitely would be Martian Manhunter. The ability to shapeshift at will and to phase through solid objects, as well as being able to fly, reading minds and shit would be so helpful in life.
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Y'know, upon further consideration, I think I'd rather be Captain Marvel over being a Prototype. Just as strong as Superman, and even tougher and faster. He doesn't have as many powers (he's pretty much limited to super strength, speed, durability, and I'm pretty sure he can shoot magical lightning), but he has no real weakness. Not even magic is a serious threat to him.