This is so hard to answer for me. I remember so many great games tbh.
Cant believe no1 mentioned Fallout 1/Fallout 2, Planescape Torment, HoMM 3. HoMM 4 was widely frowned upon but it had the best single player campaign and sountrack of all the HoMM games. If you actually read all the text like a massive nerd
Morrowind is the only game i believe i reinstalled at least 20 times. Always find something new in every playtrough, the atmosphere, the music, the modding community. Stunning masterpiece and by far the best single player game i have ever played.
Icewind dale, Newerwinter nights, Baldurs gate, Avenrum, original Dragon age were all golden, most recently enjoyed Pillars of eternnity a lot. I love all old school isometric rpgs. I wish there were more of them, If anyone has a hidden gem let me know.
Dungeon keeper 1 and 2 were AMAZING games. Warcraft 3 and TFT campaign incredible.
Dwarf foretress hard to get into, but sooooooo worth it, most intricate game you will encounter. Mafia 1, Medieval 2 total war total conversion mods were great. Mass Effect 2 was the peak, 1 was good, 3 was meh.
Age of empiers 2 ftw, Diablo and Diablo 2 were a massive timesink and fantastic games. Civ 2 was the shit at the time.
Rockstar games. Vice city, San Andreas, Red dead redemption.
Im sure i missed a lot of them, this is just what i can think of atm
Tl:dr Holy trinity Morrowind/Fallout 2/Diablo 2