well it will probably change in ten minutes (I guess you could say I am a fickle guy) but as of now
1: UFO defense/X-com ()
2: Warcraft III
3: S.T.A.L.K.E.R
4: Batman AA
5: dragon age

Quake
Starcraft
Diablo
Unreal Tournament
Myst

No particular order, and more like top 8:

Doom (II)

StarCraft

Diablo II (tie)

Planescape: Torment

Ultima VII

King's Quest VI/Conquest of the Longbow

Baldur's Gate II

1. Civilization 1 or IV
2. Jagged Alliance 2.
3. Master of Orion 2.
4. Xcom: Ufo Defence.
5. Heroes of Might and Magic 1, II or III.

Wow, do you think I'd like Valkria Chronicles then?! I suspect im more of a strategy buff than I would care to admit.

1. The Secret of Monkey Island/Monkey Island 2: LeChuck's Revenge
2. Civilization
3. Dune II: Battle for Arrakis
4. Dungeon Keeper
5. Maniac Mansion: Day of the Tentacle

1. KOTOR
2. KOTOR 2
3. Star Wars: Galactic Battlegrounds
4. Zeus: Master of Olympus
5. Rome: Total War
6. Jedi Outcast
7. Rainbow Six 3: Raven Shield
8. WoW
9. Battlefield 2
10. Counter-Strike
11. The Battle for Middle-Earth
12. Dragon Age: Origins
13. Portal
14. Sim City 2000
15. Team Fortress 2
16. Empire: Total War
17. Bid For Power(quake 3 mod)
18. Quake 3
19. Unreal Tournament 2k4
20. Sins of a Solar Empire

Picking just five is difficult. =(

1) Elder Scrolls 2: Daggerfall (buggiest game EVER on the PC that worked)
2) Knights of the Old Republic 1/2 (count as the same)
3) Warcraft 2
4) Magic Carpet
5) Wizardry (no particular one, I enjoyed all of them)

Company of Heroes
Red Alert 2
SimCity 4

(I don't play a lot of PC games haha).

Deus Ex
Starcraft
Fallout 2
Planescape Torment
Quake 3

1. Everquest
2. World of Warcraft
3. ES: Morrowind
4. Neverwinter Nights
5. Diablo 2