That's a good one.

Hitler and Stalin seem like obvious choices. What about saying Hi to John Wilkes Booth with an M1A?

Regardless of your religious beliefs, if anyone could handle a little computer commando appearing on his door with M16 without breaking a sweat, I'd say that'd be J.C.

How about appearing in front of Patten long before WWII breaks out and before the garand is invented and handing him an AR15. Of course if you startled him you might not make it back in one piece...

My first thought when I read this was a dinosaur hunt, but I guess I'd go with getting the chance to be on the island where my uncle was a radio operator before the japanese landed with almost no warning, capturing almost all of the men in his unit, most of whom died (he escaped).