mine would be a tie between Elder Scrolls Morrowind and Elder Scrolls Oblivion.
Morrowind was very much the premier hardcore rpg of its time as far as single player rpgs go.it was way more difficult than subsequent TES games in that spells could fail if your skill in whatever magic school you were using was not high enough,no compass so you had to either follow directions or figure it out yourself,etc. i still remember the gamestore employee gushing about this game and veritably shoving it into my hands back in 05 or sometime close. i still play this game btw even currently.
Oblivion was the transition between morrowind being hardcore and skyrim being really watered down IMO as it still had some complexitly but you could see that Bethesda was beggining the seriously streamline the experience. i still feel that skyrim is a great game but a not really that hardcore rpg niche that morrowind was and that TES used to represent.
Diablo and WoW are games i spent alot of time with so they get an honourable mention....