On Playstation: Threads of Fate. First game I ever finished and i love the shape-shifting capabilities of Rue.
On Nintendo 64:: Legend of Zelda: Ocarina of Time - finished it, found all the skulltulas but not all of the secret. SO much fun!
On Playstation 2: Final Fantasy XI - it was beautiful and I think it still is, just hated the grind. Also, having to kill a ton of leeches for one Carbuncle Ruby was grr!
Honorable mentions to Devil May Cry because Dante, duh and oh also, wrecking the environment - almost everything you can see, you can slash. ^_^. Also to Final Fantasy 8 for turning enemy creatures into cards - loved that game mechanic. Never completed either. T_T.
Oddity that I miss: Fairyland MMORPG, first and second. The 2nd was taken down - didn’t gain popularity but the creature designs were wonderful and I miss that world. Would love if they had left it up, just to play for fun. Nostalgic now. Thanks to the OP for the question.
Edited for clarity and highlights