Well, here's some ones to avoid..
1 - Manhunt / Any 18 Rated Game that promotes Violence
2 - Any Grand Theft Auto Game (or anything similar)
3 - BMX XXX / Leisure Suit Larry / Any Porno Game
4 - 50 Cent's Sissy Game
5 - Any TV Show / Movie Tie-In Game (well, not all of them some are good)
6 - Any Game Liscensed by Pop Stars / Other Celebrities (EG: Britney Spear's Stupid Dance Game)
7 - Any Football / Soccer Game (EG: Fifa liscensed crap..)
8 - Crazy Frog Racer
9 - Any Karioke / EyeToy game
10 - Power Rangers Dino Thunder
As for ones to buy.. I dunno, there's been quite alot of crap everywhere on PS2 recently. Although I do reccomend buying Tekken 5 (if you're into beatemups). Zone of the Enders is quite good as is Metal Gear Solid 2 (avoid the Substance version, there's a lot of worthless bonus crap on this one). Oni is quite good and you should find it cheap 2nd hand.. well that's all I can think of right now..