Morrowind. It's depth, worldbuilding, lore, and exploration manage to make the world feel alive when mechanically it is not. But it is not a game for everyone and I don't fault people for not liking it. I feel the character-based mechanics and reliance on the player's problem-solving and reading comprehensions are very much in line with the tone of the game but I can see why people don't like that. Nothing wrong with better accessibility and action-oriented gameplay, just dull lazy questwriting *cough*^Skyrim and bland shallow worlds *cough*^Oblivion

Skyrim is actually my 2nd favorite though, but that's mostly due to the massive modding community. For almost every issue I have with that game there are 10 different mods that fix it, then thousands with new content so you can pretty much build you own game. Obviously players shouldn't have to build their own damn game to have fun but that doesn't change the fact that once I have taken a few days to do so it's very close to exactly what I'd want in an openworld RPG (so uh, Viking Morrowind with modern gameplay and AI.) Besides, the worldbuilding, exploration, and gameplay were fine.