Here's my list of 50 most favorite games from more than 5 years ago: http://gamrconnect.vgchartz.com/post.php?id=3725927
So at the moment my top PC exclusives are:
1. Maniac Mansion: Day of the Tentacle
2. Age of Empires II: The Age of Kings => disqualified, was ported to PS1
3. Wing Commander Privateer
4. Dungeon Keeper 2
5. The Curse of Monkey Island
6. Betrayal at Krondor
7. Baldur's Gate
8. Lands of Lore: The Throne of Chaos
9. The Settlers II: Veni, Vidi, Vici
10. Grand Prix 3
11. System Shock
12. Anno 1701
13. Master of Orion
14. Master of Magic
15. Outcast
16. The Legend of Kyrandia: Hand of Fate
17. Full Throttle
Unfortunately I can't mention the legendary RTS game Dune II: Battle for Arrakis as this was ported to Sega Genesis acc. to mobygames.
I'm surprised by the lack of Ultima VII (plus expansions) in this thread. I've never played it but afaik it's considered one of the best CRPGs ever created.
