Morrowind + a ton of non-destructive mods. And by that I mean mods that add to the Morrowind experience without ruining it, and without stealing its focus (so no mods with lots of new stuff that doesn't fit into the lore)
Kotor is a great game but come on... you know you want to play Morrowind instead :3
This times a thousand. Not going to lie though, I'm absolutely biased, as Morrowind is my favorite and most played game of all time.