Depends on your preferences and enthusiasm for the games, I guess. Of the ones I know, Enclave seems to be pretty quick and short, while holding some replay value. I haven't played through it yet - still in the Light campaign - but it seems I'm already nearing halftime after just a couple of quick runs. Also, the story isn't all that important, if it's your type of game you can have fun playing levels independently of progression, and most levels are pretty short runs (5-20 minutes, I'd say). I'm playing on Easy mode though, could be more frustrating and slow on the harder difficulties.
If you like adventure games, Sanitarium is nice because it consists of several short story chapters instead of being an epic tale. I managed to play through it, tackling one chapter per evening.
And I second swizzle66's suggestion, MDK seems pretty short, maybe just a few hours.
Post edited July 29, 2011 by Leroux