Jak II: Renegade - Big, open city? Check. External levels to offer maximum variation? Check. Carjacking, weapons, puzzles, platforming, mini-games, a hoverboard and tons of humor while still offering a challenge? Big. Freakin'. Check.
Ratchet & Clank 2 - Thanks to a new strafing mechanic, some RPG-elements and (in my opinion) the best weapons and levels; this one still remains my favorite R&C title up to date.
Spyro: Gateway to Glimmer (or Ripto's Rage, for you Americana people) - My first PS game *tear of nostalgia*
Shadow Hearts: Covenant - Probably one of the best sequels ever. The original SH had a great story with equally great characters, accompanied by a soundtrack that has achieved cult status. Sadly, the gameplay didn't have much to offer (except for the Judgement Ring mechanic) and the graphics downright sucked. This sequel kept all the good points and fixed all the bad ones, resulting in one of the best RPGs I've ever played.
Shin Megami Tensei: Lucifer's Call (or Nocturne, again, for you USA-folks) - I remember the European reviews of this game all saying something down the line of "...like Pokémon". No. Just, no. The first Megaten game was released in 1987, the first Pokémon game was released in 1996. So Pokémon is like Megami Tensei, not the other way around. And personally, I like the Megaten series better than Pokémon. Why? Better monster designs. Better music. Better story. More mature. No bullshit with "pokéballs", you want someone in your team, you go up and ask him/her. Fusions and sacrificial fusions to create new demons. Not limited to 4 moves for each creature. Actual evolution in gameplay mechanics over the years. Etc.
Final Fantasy IX - You could write a book about why I think FF IX is a really, really good game. From the strong female characters to the amazingly inspiring fantasy world that would even make Tolkien's jaw drop. From the surprisingly heavy story to the deviously catchy soundtrack.
Chrono Trigger - Non-linear time-traveling in an RPG? Yes please.
Legend of Legaia - A gem time has forgotten. The graphics aren't up to scratch and the dialogue isn't always up to scratch, but it manages to create a unique, solid world with a strong storyline and an interesting gameplay mechanic. From the atmosphere to the combat, from the monster design to the music, it really succeeds in setting itself apart.
Xenoblade Chronicles - The reason I bought a Wii, do I need to say more?
Okami - Think "Zelda", but better.