I just finished uncharted 2 which was the game I bought with the system 2 months ago. What good game(s) should I get next?

Metal Gear Solid 4 is the best game on the system, but make sure you play 1-3 if you're going to play 4.
 
Otherwise, Demon's Souls is the second best.

There's always Infamous, killzone 2, and of course the first uncharted which I really liked(though you might find it a little underwhelming since it's pretty much the same as the second one, but in a much smaller scale, still enjoyable anyway), also if you like Story heavy anime like stuff and a little strategy you should get Valkirya Chronicles, it's great., also as fancySoapsman said, MGS4 and Demons Souls are just no brainers.

Infamous (get it seriously) Ratchet and Clank Tools of Destruction and A Crack in Time (you won't find two funnier or better games on the system). Just a few for right now. I'm sure others will fill the gaps.

Ratchet and Clank.  
God of War.

Lair and Haze.

Valkyria Chronicles

" Metal Gear Solid 4 is the best game on the system, but make sure you play 1-3 if you're going to play 4.  Otherwise, Demon's Souls is the second best. "

Motorstorm Pacific Rift. MAG/Warhawk, Disgaea 3. Joe Danger

littlebigplanet is mucho fun. resistance was good also

Dude, there's a recommended PS3 games thread here. Try that first.

" Lair and Haze. "

I would vote for Heavy Rain or MGS4. Those are probably my favorites of what I've played.