Uncharted. The only problem with the series is ... 2 and 3 are on an ENTIRELY different level than 1 ... but 1 is a ridiculously good game (far above average standards, even now) And it's sort of a disservice to it to play like 2 (the best one) then go back, it'll just seem like shit in comparison, even though 1 is still a stellar game.
But, you can't go wrong with Uncharted, unless you don't like things that ooze awesomeness. The series is sort of like the ultimate culmination of everything that's come before in platforming, adventure, and action games. It's a series of games that probably couldn't exist before now. Tomb Raider was sort of the architect of the way it plays ... but Uncharted it substantially better for a few reasons: namely the mechanics actually work and you have interesting characters too. Win-win.
Infamous is good stuff and it's pretty long ... in a good way. The game also unfolds in very distinct ways depending on the dichotomy choices you make. It is good or bad ... but the alterations are pretty staggering. It changes what characters live, what powers you get, and even various points of the narrative.
Ratchet and Clank are great. It just depends on how OCD you are about playing them in order ... considering the main 2 on PS3 are games four and five.
God of War 3 is just ludicrous crazy action. The strong-suit of the game is easily the fact that the bosses you're fighting are entire set-pieces. It just gets insane to a degree the previous 2 games couldn't dream of.
I'd say that's about it. Realistically R&C, Uncharted, and GoW games aren't really that long. I mean, if you played all three Uncharted games, both R&C, and GoW3, you're looking at like 60-70 hours of SP (though Uncharted 2 and 3 have full online coop and vs) Infamous though ... I think I put in a solid 40 hours into that game for my first playthrough. I guess it just depends on what you're play style is.
Also, the resistance and killzone titles are actually pretty good, I didn't list them, because it's just like ... oh 5 more FPS ... yeah thanks. But They're not bad. I think actually Killzone 2 and 3 are pretty unique in many aspects. They're also all very cheap. I think I bought Resistance 1 for like $6. CHEAP.
Anyway, your first purchase should be you going out the door this instant to buy Uncharted. Your second purchase should be infamous or uncharted 2