I secretly desire your console!

Metal Gear Solid 3: Subsistience - I'll be honest: I've only played the regular version (Even though I have Subsistience), but it would be stupid to pass up the superior version. This game is everything MGS2 should have been. It has a great storyline, the graphics and music are phenominal, and the gameplay is much, much better than ever thought possible. If you played MGS2 and were disappointed, this will totally redeem the series for you, it's just that good.

Devil May Cry 3:SE / God of War - I'm just gonna recommend both right here, since they are both awesome and both $20 each. It really depends on what you want more of in a game. In all fairness, God of War is likely the superior game, but I'm giving a huge edge to DMC3 for the awesomeness of it's combat engine and the insanely cool Virgil (Only applies to SE, of course). Most people probably enjoyed God of War more though. (If you had asked me a year ago, the best Greco-Roman themed action game on the PS2 was Rygar, which I seriously considered mentioning here as well. That game has some seriously great art direction, music, and overall fan-fucking-tastic gameplay.)

Drakengard - While the game itself is somewhat lackluster, the storyline is so amazing that it compelled me to get every single weapon in the game to unlock the fifth ending (Which is pretty goddamn crazy, but 4 out of 5 are anyway.). It's pretty flawed, but the only other game with a story good enough to get me to forgive flawed gameplay is Xenogears, if that's any indication.

Onimusha 3 - The Onimusha series may have one whack-ass storyline, but this game is pretty goddamn awesome. It improves on Onimusha 1 and 2's gameplay in every way possible, and it adds a new playable character to the game. Of course, most of my liking of this game is probably attributed to fandom of Jean Reno, but the game is pretty cool despite my bias. Also, the opening cutscene may very well be the best in any game ever.

Resident Evil 4 - I only put this down if you didn't play this game on Gamecube. Quite frankly, this game has had it's dick sucked all year, so I don't really need to tell you how goddamn good it is.

If you have RE4 for GCN: Shadow of the Colossus - I didn't want to mention this one since everyone in this thread has thier hand down this game's pants, but it is indeed a good game. It's really at it's best when you're climbing on a colossus and hearing the awesome soundtrack though, because it really does have great music. Ico is a good game, but it's far too hyped up, and the damn buddy mechanics are fucking annoying. Shadow of the Colossus is like Ico on steroids, or the bigger, faster, and meaner version of Ico, where the castle is alive, and your lifelong companion can actually DO something (Yorda could open doors or some shit, I know, but at least the horse doesn't get owned by shadow monsters whenever you leave it alone.)

Katamari Damacy, ZOE2, and Guitar Hero also come very highly recommended by me. The Ace Combat games are pretty cool, but I don't really see what the hell Tails and the other OBSESS about, but then again, people obsess about every game.

Jam it back in, in the dark.

Rab, MGS3, RE4, Onimusha 3 and DMC3 are all the best games in their respective series (This is subject to debate among some, except RE4.) Do you have a problem with games that have sequels in the first place? Usually games have sequels because THEY ARE GOOD, you know.

Unless you have a problem with most of my top five being action games, but that's mostly because that's the best the PS2 has to offer sans RPGs. Here's a couple diverse options for ya. (And if you must only get one action game for the PS2, I recommend RE4, unless you have it for GCN already, in which case I recommend MGS3)

Horror: Silent Hill 2 - I don't consider RE4 to be a horror game so much as an action game, but SH2 will seriously scare the crap out of you. I know it's a sequel, but the original game is only on PS1. It's a great fucking game though.

Music: Guitar Hero - Fuck all these DDR loving nerds, this game is fucking amazing. If you like rock music, this is the music game for you. I don't think I've ever played a rhythm game as insanely awesome as this one. It's really something else.

FPS: Killzone - This game may suffer from hype fever, but it's a pretty solid FPS with some pretty impressive graphics for the ol' PS2. It's either this or Timesplitters: Future Perfect, really, but that's on the other two systems as well. (And it's part of a series. Oh no!)

Racing: Gran Turismo 4 - The racing itself is spot on, but once again the sheer number of options in this game is incredible. One of the few non-RPGs where you can spend hours and hours just going through menus customizing shit. It's overrated, yeah, but that doesn't stop it from being awesome.

Fighting: Virtua Fighter 4: Evolution - The deepest fighting game ever, bar none. No other fighting game will truely match the maddening depth this game has. This isn't like DOA where any little pussy can play: This is hard-core fighting at it's best. I know this may technically be an action game, but half of games nowadays are action games anyway.

There's nowhere I can't reach.