I ran an Edge of the Empire one shot for about two hours. That worked pretty well. It was for my teenaged nephews who are dabbling with RPGs. My kiddos were also part of that game and it was apparent that mine are natural improv roleplayers while the other two (who are older than mine) where uncomfortable and restraining themselves. It was truly refreshing.
I feel like the lighter the system, the more apt for a two hour session it would be.
Any system with people who already know that system, would work for a two hour session.