No particular order (except maybe the top 5).
System I played them on in parenthesis
Zelda: Ocarina of Time (64)
Baseball Stars (NES)
Super Mario Bros. (NES)
Super Mario Bros. 64 (64)
Command & Conquer: Red Alert (PC)
Heroes of Might and Magic 2 (Mac)
NHL 96 (PC)
Test Drive (PC)
Super Mario Cart (SNES)
Age of Empires II (PC)
Civilization II (PC)
Sonic the Hedgehog (Genesis)
Warcraft II (PC)
Pacman (Atari 2600)
Call of Duty 4 (XBOX 360
This was really hard actually.
Tecmo Super Bowl
Super Mario World
Battletoads
Tetris
Baldur’s Gate 2
Planescape: Torment
FF III
Half Life 2
Team Fortress 2
Diablo 2
Medieval: TW
Operation Art of War III
World in Conflict
Starcraft
Civ 4
Crystalis (NES, if you have not played this game its amazing, its like the best Zelda game times 50)
Knights of the Old Republic
Morrowind
Super Mario Brothers 3
Quake 2 (online death match has still not topped this game)
Halflife
Age of Empires 2
Civ 2
River City Ransom (this game holds a special place in my heart)
Final Fantasy 3/6 (the series peaked here, not that some of the others are bad this is just the best yet)
Thats only ten, but they are amazing games.
My favorite 15 games of all time? Tough.
In no order because it’s a stream of consciousness here:
Doom
Super Mario Bros
Zelda
Civilization
X-Com UFO Defense
Unreal Tournament
Knights of the Old Republic
Never winter Nights
X-Wing vs Tie Fighter
Starcraft
Tetris
Pharaoh
SimCity
Grand Theft Auto 3
Need For Speed 3
Why yes… I’m old school. Why do you ask?
Seriously though, those games are mostly older games that I feel have proven the test of time for me. I can and do load them up from time to time still and enjoy playing them again, years after they were released. There are a number of ‘could be’ contenders out there, but I won’t really know how relevant or awesome they are until I see how much I still like them in a few years. They’re good, but greatness (IMO) comes from the ability to remain relevant and enjoyable long after the shine has faded
I couldn’t even begin to select a top fifteen; my favorites exist in a cloud of potential and a “top 15” wouldn’t be consistent from hour to hour. Twenty years ago I might have been able to select a top fifteen, these days it would have to be a top two hundred and then I’d feel like I was missing something.
In no particular order, besides maybe the number of hours I’ve spent on each:
Counter-Strike (I honestly believe I screwed up an education because of this game)
Morrowind (with mods and extensions)
Neverwinter Nights
Fallout 1, 2, Tactics, 3 (Yes, I like Tactics.)
Gothic (Hard for me to distinguish between 'em)
GTA: Vice City and San Andreas
Need for Speed (one of them)
Starcraft
I add
Nobungas Ambition
Genghis Khan
I could only come up with 14:
Dungeon Keeper II (pc)
American McGee’s Alice (pc)
Grand Theft Auto III (pc)
Rockband (ps3)
Cell Damage (Xbox)
Tempest (pc)*
Centipede (pc)*
Defender (Atari 2600)
Demon Attack (Atari 2600)
Sims (pc)
DOA (Xbox)
Tetris (pc)
Project Gotham (Xbox)
Civ III (pc)
*From the Atari classics reissue, which is pretty much on par with the original arcade version.
Startopia should be up there, but I never finished it. I plan to give it another attempt once I finish Dragons Lair 3D-a game that promises to be the worst piece of crap I have installed on my pc.
Archon
Planetfall “why you do that to Floyd?”
Wing Commander III Heart of the Tiger (Mark Hammil, man)
Half Life
Castle Wolfenstein (original 2d commodore 64)
Return to Castle Wolfenstein
Janes US Navy Fighters
Balders Gate
Crusader
Knights of the Old Republic
Jedi knight (any of them)
Nobunagas Ambition
Half Life 2
Full Throttle
Diablo
My best games ever (in particular order):
Guess which generation of games I grew up in? 
Although I’m seriously contemplating replacing Einhander with Persona 4 because it was just that awesome.
After thought, I’d like to put in the Madden series, The NHL hockey series, Sim City 4, combine Chrono Trigger with Secret of Mana, Romance of the Three Kingdoms, and Tetris.
I hate forgetting something, but I know I am.
X-com
Baldur’s Gate 2
Half-Life
Half-Life 2
Team Fortress 2
Super Metroid
Zelda 64 OoT
Starcraft
Warcraft 2
Diablo 2
Knights of the Old Republic
Halo
Fallout
Dead Space
Resident Evil 4
Not all of these have aged well, but I’m going to parse this as my top 15 game experiences. No particular order:
Morrowind
Final Fantasy VII
Dragon Warrior IV (NES)
Team Fortress 2
Baldur’s Gate 2
Counter-Strike
Castlevania: Symphony of the Night
King’s Field: The Ancient City
Diablo II
Planescape: Torment
Civilization: Alpha Centauri
Masters of Orion II
Mario Kart 64
Age of Empires II
Star Control II
…and an honorable mention to all those old TSR D&D games like Champions of Krynn.
My picks…many are out of respect for their impact at the time, or their defining a baseline for a successful genre of the future.
Baldur’s Gate (series)
Civ: Alpha Centauri
Tie Fighter
Soul Calibur (series)
Diablo 1 (sorry folks, D2 didn’t have the fun or charm).
Elder Scrolls: Oblivion (warts, but moddable to greatness)
Sim City 2000
Wing Commander 3
Wizardry
Sonic the Hedgehog
X-Com
Red Baron
Total Annihilation
Doom
Mario Kart