It's difficult to say, because some games that I have loved over the years are a little difficult to get back into today. For example, I loved Final Fantasy VII back in 1998, but I got it on PSN and haven't been able to play further than an hour into it.

So, these are my top 10 best that I never get sick of to this day:

1) Zelda: A Link to the Past
2) Super Metroid
3) Castlevania: Symphony of the Night
4) Super Mario World
5) Super Mario 3 (or All-Stars if possible)
6) Streets of Rage
7) Illusion of Gaia
8) Fallout 3 (or New Vegas)
9) Eternal Champions CD
10) Super Castlevania IV

I just recently got back into PC gaming, having become an active member at GOG, I'll have to go back and play some of my old PC favorites (Fallout 2, Deus Ex, Planescape Torment, etc) before honestly adding them into this list.

improtu 10:

1. Go
2. Mahjong (chinese/japanese)
3. Blood Bowl
4. pnp rpgs in general
5. MUD's ( Icesus/BatMUD etc)
6. Total Annihilation (multiplayer)
7. COD2 -era clan matches
8. Tie-Fighter (played it before X-Wing)
9. I-War 1
10. Scubaball

I simply can't come up with my 10 favorite of all time -- I'd have to do my top three of each genre for it to be even slightly accurate! However, I can list some of the games that had the biggest impact on me:

Civilization 2
Zoo Tycoon Complete Collection
The Walking Dead Season 1
Cognition
Donkey Kong 64
Wild Arms 3
Star Ocean: Till the End of Time
Doorways (between Walking Dead and Doorways, I've entered a whole new realm of games!)
Mystery Case Files: Madame Fate and Midnight Mysteries: The Edgar Allan Poe Conspiracy (these are the two games that drew me into casual gaming)
Ys Book I & II - cemented my love of RPG's

Wahaha somebody needs to create a new top10 pronto with that in the 1.) position otherwise he'll track down everyone on this thread

edit : in all fairness the only reason its not on my list is because I haven't played it yet :P I want to play Giants first

edit 2 : I am delighted to see though that a lot of people have Fallout 2 on their list

In no particular order, but I love all ten of those very very dearly:

Deus Ex
Mafia
Commandos: Behind Enemy Lines
Hitman: Codename 47
C&C: Tiberian Sun
Grim Fandango
Broken Sword: Shadow of the Templars
Donkey Kong Country 2: Diddy's Kong Quest (SNES)
Carmageddon
Unreal Tournament
Balls of Steel

(yes, I put eleven, so sue me, it's a holy number)

Honorable mentions:
Other games in the same series as those mentioned above for the most part
Ubisoft's Prince of Persia Sands of Time
Monkey Island series, especially the third one though
Splinter Cell
Desperados: Wanted dead or alive

10) Driv3r
9) SimCity 2013
8) Resident Evil 6
7) Call of Duty Ghosts
6) Lair
5) Haze
4) Aliens Colonial Marines
3) Daikatana
2) Duke Nukem Forever
1) Deus Ex 2 Invisible War

Notice the first 3 start with D.

Yeah, I'm thinking the rage factor could be dangerous....

So I'll do a new top 10

1) Sacrifice
2) Sacrifice
3) Sacrifice
4) Sacrifice
5) Sacrifice
6) Sacrifice
7) Sacrifice
8) Sacrifice
9) Sacrifice
10) FOUR GAMES TIED FOR 10th PLACE
10a) Sacrifice
10b) Sacrifice
10c) Sacrifice
10d) Sacrifice

HONORABLE MENTION

Sacrifice
Also, too, Sacrifice.
Oh, and Sacrifice.

EDIT: Oh I almost forgot this one... Sacrifice.

Uh...huh? Is this list a joke? If not, apologies.

Anyway, my list, not in order, as it is hard enough to narrow it down to ten. Limited myself to one per franchise or it we a list of Zelda/Mario/Metroid games.

Age of Empires II (PC)
Chrono Trigger (SNES)
Final Fantasy VI (SNES)
Grim Fandango (PC)
RollerCoaster Tycoon (PC)
Super Mario Bros. 3 (SNES)
Super Metroid (SNES)
System Shock 2 (PC)
The Legend of Zelda: Ocarina of Time (N64)
The Operative: No One Lives Forever (PC)

PDS games, in reverse order of release.

Oki, after some deliberation

1.) Blade Runner
2.) Fallout 2
3.) Wing Commander IV : The Price of Freedom
4.) Civilization 4
5.) Outcast
6.) Homeworld
7.) Transport Tycoon Deluxe
8.) Warcraft 3
9.) Half life
10.) Anno 1602

I can't think of anything that can bump off those games, although some games come terribly close : Red Alert, Dungeon Keeper, Dune 2, Half-life

....

1.Ride To Hell
2.Mindjack
3.Happy Wheels
4.Slenderman
5.Deadly Premonition
6.Goat Simulator
7.Bear Simulator
8.Man Vs. Wild
9.Halo 3:ODST
10.Big Rigs

Advance Wars
Another World
Cave Story
Day Of The Tentacle
Dungeon Master
Final Fantasy VII
Full Throttle
Grim Fandango
The Journeyman Project 2: Buried In Time
La-Mulana
The Legend Of Zelda: Ocarina Of Time
Max Payne
Portal 2
Super Mario World
Super Metroid

Ah I've got it. Although it saddens me to remove a childhood hero, I have to :

1.) Blade Runner
2.) Fallout 2
3.) Wing Commander IV : The Price of Freedom
4.) Civilization 4
5.) Outcast
6.) Homeworld
7.) Kerbal Space Program
8.) Warcraft 3
9.) Half life
10.) Anno 1602