I'd recommend the following:
MGS4 is a must if you're thinking about it
Call of Duty 4 & World at War, though 4 may be a better place to start
Fallout 3, if you can live with a lot of bugs and crashes, its a huge game and fun if it works.
Far Cry 2 is quite fun if you can get it cheap
Little Big Planet
Burnout Paradise will last you a while
Resistance 2
Bioshock
GTA IV.
All of the above have kept me playing for ages, and so you have an idea of the kind of scale we're talking about here, I finished Assassin's Creed in a weekend (as in all the missions, not all the flags and Templars... Which was pointless.).
Wipeout HD, Warhawk, Super Stardust HD and Burnout (if you don't get it on disc) are the only PSN games I'd bother with, personally, Pain really wasn't worth it as far as I'm concerned.
But yeah, if you want some games that'll last you some time (and you like online play, that is), go with those!
Edit: Uncharted is also quite good, but not all that challenging, even on Crushing. Getting all the trophies is really the only replay value (all the in-game achievements are the same) and I got all of those in a weekend too...!
Edit again: I don't mean to boast, just saying it might not last that long. If you can get it cheap though, do it, otherwise maybe put it further down the list?