What game should I get?

It would help if you gave some idea what you already have and have played. A decent, similar thread for PS2 recommendations is RAB's recent one:
http://www.gamingforce.com/forums/so...r-console.html

I second chao's recommendations for ICO and Katamari.
Both Katamari Damacy games are essential PS2 purchases. Quirky, extremely funny and brilliant in design and execution, not to mention awesome soundtracks. They have the kind of respect and love for games and gamers alike that are so missing from most console titles these days, and they remind you why you play games in the first place.

ICO is the very definition of gem. Relatively small in scale, but beautiful in execution (and I don't only mean the graphics). A wonderfully elegant title and different take on the platform adventure. Also my personal favorite console game ever and in my opinion the Playstation 2's finest.

From GBA, I'm currently going through Legend of Zelda - Minish Cap and it's absolutely brilliant. I was worried it would feel tired and calculated, especially after replaying the magnificent Link to the Past, but Capcom's take is wonderful, familiar enough to feel safe, yet new and clever enough to feel fresh and costantly surprising you pleasantly. It's a terrifyingly addicting game though, so be warned.

Before Minish Cap, I finished Mario & Luigi - Superstar Saga, which was awesome. One of the funniest games I've ever played, a wonderfully witty and clever action-RPG, that loves games and Nintendo, but is not afraid to poke fun at either. If you've played your fair share of Nintendo classics, there's a massive amount of humor to be discovered in the story and settings, constantly embracing and making fun of Mario cliches and video game conventions alike.

For PC, I seriously recommend playing Grim Fandango, if you haven't yet. A brilliant, extremely stylish and witty adventure game, with a superb script, cast and soundtrack. Some of the greatest dialogue in games ever, and my favorite PC game. Some of the puzzles can be a bit obscure, but they all consistently follow the gameworld's logic.

Jam it back in, in the dark.

Originally Posted by Elixir

But I'll third Katamari. Once you've completed it, buy We Love Katamari, as you won't find yourself playing the original again. The second basically takes the first, upgrades it, and does everything right. It's a great sequel and has an incredible amount of replay value.

I hope you realize you're basically saying "the first game isn't that good, you should only buy the second". Which of course is blatant lying and the kind of ignorance that should be punished with multiple lashes on the ear.

There's nowhere I can't reach.