For many years, I thought that KOTOR was the greatest game ever made with a huge amount of rich dialog, characters you cared about who joined your party and interacted with each other, a really interesting plot with a fantastic twist that really hit you in the gut (not quite as jaw-dropping as Bioshock's, but close), the ability to play as light or dark and the meaningful way those choices changed the game and character interactions... And based on the original and still the best pen-and-paper open gaming d20 system, just an amazing game. It held that crown until I played CDPR's The Witcher 3 and then Cyberpunk 2077, which are now my top 2 games of all time, with KOTOR still in the top 3. Obviously, given the similarities between those, I'm an RPG guy first and foremost.
Other great games in that list would be:
Ultima IV - a breakthrough game, really the first RPG with a meaningful story and where it felt like decisions and choices mattered. First game where you were rewarded for being ethical (long before KOTOR) instead of just for killing everything (but there was plenty of that kind of action too).
Morrowind - the vast and interesting open world and incredible depth to the lore, the impact of choosing which family to align with, all brilliantly done and felt more meaningful than Oblivion or Skyrim
Skyrim - for all the reasons that everyone knows this game was great, but also the first RPG that, at least for me, provided enough variety of things to do (minigames, diverse areas to explore), that the game never really got boring. Fallout 4 was even better at that, but it had other problems that keep it off my "best games ever" list, plus Skyrim had this attribute first.
Wizardry I - no other game has had me feel so attached to my characters, where every small gain of a hit point, new spell, or ability felt incredible, and all deaths were permanent unless someone else survived to bring back the character (yet there was a risk of being turned to ash and lost forever). Zero save scumming, it was completely impossible -- all events were permanent to anyone else who every plaid the game on the same disk. Unique weapons, armor, and magic items were literally unique. If one player found one of them, no one else could ever get it again (without buying another copy of the game). No game since has had that: you played it on the original disk (copy protection made it very difficult to make a backup of the original), and the game modified the disk as you played, so there was literally no way to start over or undo anything, and the gameplay and character leveling all fit perfectly with that. If your party died out on an adventure, their bodies would lie there with their loot until someone else found them and looted them or brought back their bodies to attempt a resurrection.