Given Unlimited Time, I'd play/GM...

[Xombie Master]

My dream game: Call of Cthulhu, taking place in Russia in 1920, where the characters are members of an early Soviet expedition to Tunguska ...

Sounds pretty sweet!
You could take a little side trip to siberia to be carried away by Ithaqua.

 

[The_Universe]

Next on my list are (tied) Mutants and Masterminds 2 and Star Wars D20. I think I could sate myself with a short campaign of M&M, but I'd like to do something longer-form with Star Wars. Other than that, I already got my group to try Old West/Victorian Dark*Matter using D20 Modern and Sidewinder: Recoiled, and that's working out splendidly. Eventually, I'd like to go back and play/run D&D, as well.