In alphabetical order:
Baldur's Gate
Betrayal at Krondor
Deus Ex
Fallout
Gothic
Lands of Lore: The Throne of Chaos
Mass Effect
Star Wars: Knights of the Old Republic
System Shock
The Elder Scrolls III: Morrowind
It's a shame that I never played a Wizardry, Might & Magic or SSI RPG and only # VIII from Ultima main series (which I liked, but not enough to list it in my Top 10). In the early to mid 90s I only played few RPGs and focused on other PC genres: point&click adventure (LucasArts/Westwood/Sierra), RTS (Dune II/C&C/AoE/WarCraft/Syndicate/Dungeon Keeper), TBS (Civ/Master of Orion/HoMM/Battle Isle), building/economic sims (SimCity/Settlers/Patrician), space/flight/racing sims (WC: Privateer/X-Wing/Comanche/Microprose's F1 Grand Prix). You know, all those great genres which were once mainstream on PC, but are called "niche" or "dead" today.
I think there's a chance that one of the acclaimed earlier Ultimas (V, VI, VII, VII/2) would be among my most favourites today, same goes for Wizardry VII and Baldur's Gate 2. I still plan to play these RPG classics some day but I'm not sure if I will like them as much as I probably would have if I'd played them at release or only shortly after.
