Well, this is easy: Mystique of course! See that leg and her jumping out of that window!!!
She'll probably be the main female character (she's Jennifer Lawrence), so she needs epicness. She always was a show stealer on the action front in the other films (aside from First Class). The fact that she dared to enter Stryker's military facility on her own with only her body as a a weapon makes her a bad ass.
Storm had her moments taking out Quills and Arclight like they were nothing and then zapping Callisto in X3. But, because of her pregnancy I think she'll attack from a distance like with the tornadoes in X2. It can be epic if she takes out a few sentinels.
Rogue deserves an epic action moment, but beside being an important element to the plot, I'm not expecting her to do a lot of action. It would be quite interesting though to see her battling a sentinel.
Blink will probably have her moment, but we've seen all ready with Nightcrawler (a true epic moment) and Azazel (not too shabby either).