It sounds like you like rhythm games, and Amplitude (by Harmonix, the same company that made Guitar Hero) is the best one on the PS2. You can pick it up for like $10, too, and it doesn't require any special peripherals.
My other "must-plays" for the PS2:
-Devil May Cry 1 & 3. Excellent, highly polished, cinematic action games.
-ICO. Pretty much a fan favorite here at VGC. It's sort of like a more atmospheric, albeit linear and short, Zelda-like.
-Odin Sphere. A deep and LONG beat 'em up with heavy RPG elements. Think Valkyrie Profile, only 1000x as pretty.
-Okami. Speaking of pretty games, this is another Zelda-like that's absolutely gorgeous. You may want to wait, though, as an improved Wii port is coming out soon.
-The Ratchet & Clank games. Hybrid shooter/platformers that are loads of fun.
-Shadow Hearts 1 & 2. Unique RPGs with a fun, twitch-based combat system and quirky humor. (Skip 3, though, as it sucks.)
-Shin Megami Tensei: Nocturne, Digital Devil Saga 1 & 2, and Persona 3. The best RPGs for the PS2, by far. They're dungeon crawlers that are relatively light on story, but heavy on depth, atmosphere, and customization, not to mention difficulty.
-Silent Hill 2 & 3. But only if you can appreciate a good scare. :)
A few games that others will mention but which I'd caution against: Kingdom Hearts (a dull X-mashing fest with a story that reads like a bad fan mock-up); Disgaea and anything else by Nippon Ichi (hope you like grinding); Final Fantasy XII (the game basically plays itself and the story is incomprehensible.) But, of course, that might be just me.
"'Casual games' are something the 'Game Industry' invented to explain away the Wii success instead of actually listening or looking at what Nintendo did. There is no 'casual strategy' from Nintendo. 'Accessible strategy', yes, but ‘casual gamers’ is just the 'Game Industry''s polite way of saying what they feel: 'retarded gamers'."
-Sean Malstrom
