Tiger Woods 2003 is the best game for the entire system in my opinion. My wife is a casual golf fan but we find outselves playing it most--staying up late sometimes to play an extra 18 (we have no life obviously). I didn't like the game on the PC because it was too easy but on the PS2 you earn money with each round won, hole in one, green in regulation and you can upgrade your golfer by spending money.
ATV Off-Road Fury rocks
EA Sports March Madness (I've got 2002) also rocks. It's your standard basketball, not many frills, but it's a great basketball game in my opinion--the critics' reviews were mixed on it though.
Stay away from 989 hockey games--rented a couple and they were awful