Tons of excellent games already mentioned so I'll try to fill in some gaps:
Baldur's Gate: Dark Alliance 1 and 2 - Diablo "clones" action-RPGs similar to Gauntlet and... Diablo
Champions of Norrath and Champions Return to Arms - Sony's attempt to do the same thing as above, but in the Everquest world
Valkyrie Profile 2: Silmeria - still one of the best-looking PS2 games
Ring of Red - plays like a prototype of Valkyria Chronicles. The world setting is even kinda similar.
Dark Cloud 1 (I didn't like 2) - town building and dungeon-diving. The formula has aged very nicely.
Drakenguard series - if you wanna see the fever-dream foundations of the Nier games, here ya go.
Front Mission 5 - grab a translation. Best game in the franchise and one of the best SRPGs ever made.
Dodonpachi Daioujou - a bit short for an RPG (only 5 stages) but the grind to get your Player level up to 100 million is real.