I'm not so much talking about RPGs here, although if you're one of those folks who can lay hands on the Excalibur 2, godspeed.
At a 24-hour video game party last fall, I sat down and blasted through Mega Man Legends in 2:18, and I'm confident I can do better. At another gamer festival this past weekend, I sat down intending to see how quickly I could beat Spyro the Dragon, but got distracted by one of the bands. I've also beaten Super Mario World in 17 minutes, which I think has to be pretty close to a record since I took the shortest possible path and didn't die.
What games can you sit down, pop in, and blast through in a matter of minutes/hours?