[Crazy Jetty]

Powergirl and She-Hulk (Goddess, I would KILL to see amanda conner draw Shulky!)

Teen Titans and Runaways

Wonder Woman and (Marvel's) Herculese

Ultimate Spiderman and Powergirl would be a blast.

Trying to stay away from the direct parallels, like Green Arrow and Hawkeye. Those kinds of crossovers are about as boring as it gets.
Also, going under the assumption that all these would be out of continuity, and avoiding any migrain inducing universe hopping.
Crossovers are at their best, when they're at their simplest.