A top 10 is really difficult. I could easily change my mind every week, but here's what I'm thinking right now ...
Broken Sword 1 (PC)
Striker (Spectrum)
Elasto Mania (PC)
Flashback (Amiga)
Return To Mysterious Island (PC)
2 Player Super League (Spectrum)
Broken Sword 2 (PC)
Prince Of Persia 1 (Amiga)
Yager (PC)
Grand Theft Auto 1 (PC)
:D

I couldn't do the top 3 just 'couse it was to hard to choose only three in a list. So here's my top ten in no particular order, but chose them mostly by most influencial.
Wizardry VII
Blaster Master
Dune II
X wing
Duke Nukem 3D
Doom II
Deus Ex
Max Payne
Homeworld
Ultima Underworld

I guess my top ten games are the ones that I kept playing for months non-stop:
1. Dune 2
2. Quest for Glory (all of them)
3. Dune
4. Duke Nukem 3D
5. Oblivion
6. Baldur's Gate 2
7. Rollercoaster Tycoon
8. Outcast
9. Divine Divinity
10. Quake

1. Pyro 2 - World Terrorism (PC)
2. Slicks'n'Slide (PC)
3. Rogue (PC)
4. The Great Escape (Amstrad CPC)
5. Rebelstar Raiders (Spectrum)
6. Commando (Amstrad CPC)
7. Europa 1400: The Guild (PC)
8. Sea Dogs (PC)
9. Triplane Turmoil (PC)
10. Pro Evolution Soccer 5 (Xbox)
And so many others.. The list would be different if asked on another day, I'm sure.

1. Fallout 2 (PC)
2. The Legend of Zelda: OOT (N64)
3. Super Mario 64 (N64)
4.The Longest Journey (PC)
5. Half Life (PC)
6. Beetle Adventure Racing (N64)
7. Day of the Tentacle (PC)
8. Total Annihilation (PC)
9. Star Wars: KOTOR (XBOX)
10. Goldeneye (N64)

1. Neverwinter Nights 1 & expansions
Not so much for the original campaigns but for the many, many excellent free community-made modules. I've played 126 of them, many of them giving over 30 hours of gameplay - by far the best value for money I've ever had for any game ever.
2. Diablo 2
Of course. I even wrote spot reports about it on USENET back in the day.
3. Vampire the Masquerade Bloodlines (with community fixes)
A broken game rescued by the community. Tons of roleplay and replay value and a cool plot.
4. Bungie's Myth series (Myth III not so much)
I never liked the race to gather resources in RTS games. Here you must manage the forces you're given. Plus excellent single-player plot.
5. No One Lives Forever
I just loved the humour, snarky dialogue and 60s feel and music. NOLF2 was also good.
6. Tomb Raider (except Angel of Darkness)
The perfect 3D platformer, with guns.
7. American McGee's Alice
Love the dark twist on a classic story here.
8. Temple of Elemental Evil (with community fixes)
The only turn-by-turn D&D game that is truly faithful to the P&P game.
9. FAKK2
I just love the "let's go totally whacky" feel it has.
10. Bloodrayne
I'm always a sucker for vampire stories, plus lots of snarky dialogue and I love the tons of cool moves you can make Rayne pull off.

1. Fallout 2
2. Fallout 2 (This game is so awesome it holds 2 spots for me)
3. Day of Defeat (Counter Strike mod I played 6-7 years)
4. Panzer General 1-2
5. Ghenghis Khan
6. Ports of Call
7. Bruce Lee
8. Zorro
9. Montezuma's Revenge
10. Airlines

Wow, all these posts and XCom only made one list? One of the best games ever. And Terror from the Deep is fantastic

X-COM: UFO Defense should be on every list.
Except for Brits. They should have X-COM: Enemy Unknown on their list instead.

1. Civilization 2
2. Europa Universalis 2
3. Hearts of Iron 2
4. Baldur's Gate
5. Morrowind
6. Fallout
7. Baldur's Gate 2
8. Lost Odyssey
9. Unreal Tournament
10. Medal of Honor: Allied Assault
First 3 strategy games because I've spent thousands of hours playing them. The next three RPGs for the WOW-factor ( BG and Fallout were my first RPGs, Morrowind I loved for the freedom you have in the game). Then BG2 and LO, two solid RPGs. And finally UT and Mohaa because they were the first shooters I played online that had more interesting game modes than Deathmatch of CTF.

For me, my top 10 list would contain: (in no real order, I like them all a lot)
1. Myth I/II
2. Ultima series
3. Sanitarium
4. Syberia I/II
5. Ultima online (best mmo imo)
6. Severance: Blade of Darkness
7. Rune
8. Half life
9. Baldur's Gate
10. Mechwarrior series

Fallout series
Deus Ex series
Vampire Bloodlines
The Witcher
Hitman series
Metal Gear Solid series
Gothic series
Fahrenheit
BioShock
SW Kotor series

No particular order.
1. Half-Life
2. Half-Life 2
3. Portal
4. Team Fortress 2
5. Sam & Max: Reality 2.0
6. Battlefield 1942
7. Darwinia
8. Command & Conquer: Red Alert
9. Team Fortress 2
10. Tekken 3

No order:
1. Half-Life series
2. Portal
3. Heroes of Might and Magic series
4. Diablo series
5. Starcraft
6. F.E.A.R. series
7. Max Payne 1 & 2
8. Fallout Series
9. The Witcher :]
10. Pro Evolution Soccer

1- Thief Series (specially the I and II) - replayed those 2 games quite a few times.
2- System Shock I and II (didn't buy Bioshock due to DRM -- Although I heard they removed the limit on installations)
3- XCom I, II
4- Ultima Series (Includes II-IX, and Ultima Underworld I, II, Martian Dreams etc etc -- saves space ;p -- oh and Ultima V - Lazarus.. a remake.. liked it a lot! )
5- Might and Magic series (most of them including the HoMM series)
6- Magic Carpet I, II
7- Doom I/II + their descendants (Heretic, Hexen etc)
8- Diablo I/II/LoD
9- Starcraft II+exp
10-Warcraft 2 (not III) + exp if it had one.. can't remember right now and CDs are lost in boxes somewhere ;p
There are others like StarFlight I (very first PC game I ever played.. still find it amazing story and adventure wise).
Didn't put Fallout there since I remember vaguely playing one of them but it's been too long.. will purchase them here and play them again though.
And finally, I'd be remiss to not mention honorable mentions to Wizardry series (most of them) and Future Shock (Terminator game by Bethesda).
yikes! How could I forget Baldur's Gate II + exp & Torment!?! put it there somewhere ;p