Oooh....that's a tough one. The PS2 had a great number and variety of fantastic games, but If I had to pick one that I enjoyed heads and shoulders above all others....well I can't. Here's soem of my top games though:
Gladius: Turn-based strategic gladiatorial combat rpg in a fantasy Rome setting. Lots of customization and loot collection, with tons of different classes to recruit to your gladiatorial school, from Legionaires to Bears, Satyrs Cyclopses, Amazons, Undead.... and on top all of that off, it had two campaigns! It was an amazing game that unfortunately never got near the attention it deserved. While a sequel was briefly in the works from what I understand, all work on it was halted due to poor sales, and what could have been a great series was cut short before its time.
Star Wars Battlefront/Battlefront II: A wonderful series, it lets players indulge in battle between rebels/imperials or sepratists/clones on a number of maps, in space and on ground. I played both of these things extensively, and got quite good at them. One of my most memorable battles involved getting Darth Vader the 'bantha fodder' award at a match's end by running him over with a speeder bike numerous times.
Champions of Norrath/Champions of Norrath: Return to Arms: Two great hack and slash games for the PS2. They gave the character a massive degree of customization options, lettign them pick between male and female for every class, giving a good variety of facial customization options, and a great amount of armor and weapon customization. Personally, I prefer both the mboth to the Diablo series in just about every way. The sequel improved upon the first in several ways by giving more classes, new armor and weapons, and the option to play a through a 'good' or 'evil campaign. Although the game didn't have the plot or the flow of the first, I wholeheartedly recommend them both.
Baldur's Gate: Dark Alliance I & II: Set in the D&D setting of Forgotten Realms, these two games have little else to do with the computer games of the same name. However, they both have a great charm all their own. Personally I prefer the first. While the armor, weapon, and class selection in the first game were lesser than in the sequel or in the Champions of Norrath games, I think the first Baldur's gate game has the best crafted narrative and flow of all of them. The game takes its time, slowly progressing you further and further down into dungeons, caves, up mountains, etc. It really feels like a dungeon crawl in a lot of ways, and while you can't upgrade weapons like in the other four games, I think it makes each individual drop more unique to the player by doing this. The sequel provided more classes, such as a dwarven rogue, a drow monk, etc that were a bit more distinct than the oens from the somewhat generic oens from the first game, each with their own personality and side quest, however, the game felt noticably shorter, and the levels felt much more like levels than places ona journey, much like my complaint of the second Norrath game. Still a great game, I was looking forward to a third game in the series, sicne it had ended on a cliffhanger, but none followed. Unfortunately, with the arrival of 4th edition D&D the entire subcontinent that was foreshadowed in the ending has been retconned from existance, so I don't see a BG: DA III coming out anytime soon, which is a shame, for I liked that series (and Champions of Norrath) better than any other hack and slash series that has come out this generation.
Breath of Fire: Dragon Quarter: A huge departure fro mthe previous four games, this entry undoubtedly killed the franchise, but take for its own merits, its not necessarily a bad game. Defiantely one of the mroe challenging rpgs I have played, it was rather simple on the surface, but was progressively more difficult, and unlike many other game of the era, it punished the player aggressively for failure. Dying would cost you your exp, at the very least, and saves were limited to save coins that the player could find wandering around. Many of the later boss fights were brutally hard, and there was an exceptionally difficult side dungeon to explore. In some ways it could be seen as pioneering the paradigm of games like Dark Souls a console generation early.
