From best to...not as best:
Crackdown
R6 Vegas
Saints Row
Dead Rising
Call of Duty (2)
Lost Planet
All of those games are worth your time, regardless of how I ranked them.
Of those, you can probably get through the main story of Crackdown in a couple days if you say at it for long periods of time. Call of Duty 2 won't take you really long either on the default settings...but if you play it on the harder difficulties, it'll take you a while.
Saints Row takes a while, but it's fun. Have only briefly played Dead Rising, Lost Planet, and R6 Vegas so I can't comment on how long they'll take.