All I've played that may be considered hardcoe/old-school is Baldur's Gate, which I unfortunately never beat, but I made it significantly far into the game. I really don't know why I stopped, and my thoughts of how much I loved it and my considering starting to play it again inspired this topic haha.
pick it up again. BG2 is better in a lot of ways - the characters are a LOT more interesting, and Athkatla's level design is [censored] gorgeous - but the main quest is a little less interesting and it sort of railroads you along to different (generally pretty) areas, whereas the first game left a lot up to you to do on your own. if you dig the combat, Icewind Dale and Icewind Dale II are both really really good games - they're kind of light on the RPG stuff, but they're really solid party-based dungeon crawlers with incredible art direction and what little writing there is is really good.
Fallout 1 is an incredible game along the same vein with really good writing and world design and a ton of atmosphere, but it's a lot more open-ended than BG. the combat is turn-based and while it's pretty terrible compared to something like Jagged Alliance 2, is still kind of fun. Fallout 2 has a lot more content but i felt the overall quality of the writing and atmosphere suffered tremendously, though the world design is still pretty good.
i still haven't gotten around to playing Planescape but lots of people praise that very highly. it's an Avellone joint, though, and i'm not really big on his stuff.
Jagged Alliance 2 is pretty much the pinnacle of turn-based combat in a WRPG, and there's a lot of content. it's more of a strategy game than the other stuff i've mentioned but it's totally worth checking out if you like Fallout's combat.
if you can get over the really weird camera angle, Ultima 7 is an awesome game with a ton of stuff to do. i've seen people say Ultima 6 is better but i don't have any real experience with U6 so i can't say one way or the other. Ultima Underworld is great too, but it's a completely different kind of game.