Christ that's a hard one. It's honestly a multi-way tie.
Grand Theft Auto: San Andreas was easily the best GTA game (ever), with memorable characters, massive world that wasn't just one huge sprawling city, and pretty solid third person shooty gameplay (...for the time). Vehicle controls will seem a little dated, particularly when you have to fly planes about 3/4 of the way in. The rest of the game was just pure gold, though. If you've ever liked any GTA, GTASA is a must play.
Final Fantasy X is another solid contender. It's far more linear than the legendary final fantasy games, and honestly the characters aren't as deep as those in 6 or 8, but the plot is what saves the game. Most final fantasies just fall apart entirely when it comes to the plot - the third act of 7, the entire game of 8, 9 comes in bits and pieces, but X had a really solid theme of the conflict between the church and non-believers (and the influence the church has over the common folk), which was highlighted by members of your party being from different camps. Easily replayable even today, FFX has aged very well and the combat (which can be a bit of a chore in earlier FF titles) flows smoothly. Blitzball (a minigame) is also pretty fucking fun after the first few matches, and contains a roster of like 50+ players, a ton of skills, and 99 levels to play through. Imagine soccer, except underwater. And with the rules of physics completely ignored.
Timesplitters 2 was the FPS I put most hours of my life into. It looks *very* dated, even by PS2 standards, and some of the guns are just plain trash, but the level maker was almost unbeatable (for a console shooter, anyway) and it has a lot of fun multiplayer maps and game types. The story campaign can also be done solo or two player, and the harder modes are *fucking hard*.
Baldur's Gate: Dark Alliance. It gets a lot of (mostly undeserved) flak for not being... well, Baldur's Gate. It isn't an RPG at all, going instead for a more combat-driven hack n slash with elements taken from D&D but with a completely new combat system (i.e., one that doesn't make you sit around for 3 days waiting for the "round" to end so your character can attack again). The 3 characters in Dark Alliance all play quite differently, and aside from the sorceress, who really falls behind in the hardest mode since spell damage doesn't scale with gear are still pretty viable most of the way through. If you enjoy the first game, definitely pick up Dark Alliance 2, which is more of the same Hack n Slash with more class choice (...unfortunately, the classes aren't very standard, lacking a pure damage caster and a pure bow type).
Hope that helped.