Spider-man has met Batman, Superman, Invincible, Backlash (who is that guy?), Badrock, Red Sonja, and even Captain Crunch in a story that has still yet to be resolved, but it's been a while since he's had any adventures with people outside of the Marvel Universe (unless you want to count Conan).
So, if it was up to you and you didn't have to worry about any licensing issues, what characters would you like to see him team-up and/or fight.
Personally, I'd love to see a Spider-man/Ghostbusters team-up. You might even be able to talk JMS into writing it.