So, first thing's first, my mission is to go back in time to Nazi Germany, and kick Hitler in the testicle as hard as I can.
First ability I would choose is the Flash's super-speed. If I'm going back in time, of course I'm gonna have to have a way to travel through time, and what better way to do that than doing it Flash-style?
Second ability would be Deadpool's healing-factor. When I get to Hitler, he'll of course have a bunch of heavy-weaponed guards. Although I have super-speed, I couldn't be able to dodge every bullet, and Deadpool's healing-factor would come in very handy.
And last but not least, Nightcrawler's teleportation. If you've ever played a Nazi Germany video game or watched an action-packed Nazi movie, then you would know that Hitler always has a stylish way of escaping. With Nightcrawler's teleportation I would be able to teleport to him, and then kick him in the nut (he only had 1 testicle btw). Of course I could have teleported directly to him in the first place, but where would the fun in that be?