Quicksilver without question, he'll help you in both PvE and PvP. Proximas uniform makes her viable for ABX, Magneto at base is already good for ABX with a reasonable build. Quicksilver is near meta for his ABX day and he's also a monster in timeline and a great clearer for WBU for Proxima and Corvus. Also neither of the other two give you the return that Quicksilver's will. I've bought and used both, Magneto still doesn't do the damage he needs to make it worth it, Proxima has the damage but it's still a struggle to keep her alive despite the i-frames. Quicksilver has neither of those problems whilst also being insanely easy to time a damage proc. He's your boy without question. The uniform just makes him better at what he does.