I’m not a big fan of sessions shorter than three hours in length.  Most groups take upwards of half an hour to an hour just to settle in, unpack, and get themselves ready for the game.  That leaves little time for actually playing.

I find 3-4 hours is the perfect length for a session, although I have played shorter games and longer games.  If I was told I’d only have two hours, I’d most likely default to a board game rather than an RPG. I want every session I run to have a story arc.  A two hour session to me feels like it lends itself to “flash fiction” length games.  Three or more, that’s a short story, and that’s the length at which I feel most comfortable.

But okay, okay, I’ll do my best to answer the question in the spirit it was asked.  Probably the best game for me for a short session is one in which the rules are light and don’t bog down things very much in mechanics.  Any system like D&D that requires a lot of GM book-keeping wouldn’t be great.  You might get 2-3 encounters at best done in a 2 hour session of D&D.  Whereas, with a lighter system like Savage Worlds, I think you could get through a lot more game.

Two hours of combat wouldn’t feel like much, but a role-play heavy session of something like Savage Worlds Deadlands Reloaded could be satisfying.  A mystery adventure, something with a lot of questioning and talking.  Combat is basically, in almost every single session, highly decompressed storytelling.  Every action requires a roll.  Whereas, a game where the GM and the players trust each other to role-play and lean less on the dice, that’s likely to move more quickly.

For me, it’s not amount the number of dice rolls, and it’s not about the number of experience points.  A session’s value is measured to me in pure units of story.  So while I think two hours makes for a very brief session, combine it with a rapid play system, and you could do well enough.  Just stay away from Rolemaster.