What are the most enjoyable & memorable games you've ever played? I'm not cruel enough to make you pick just one.
Mine:
Beyond Good & Evil
Half-Life 2
Age of Empires II
Warcraft II
Dreamfall
Fahrenheit
Grand Theft Auto 2
Counter-strike: Source

The Infinity Engine games
Warcraft 3
Age of Empires 2
Half-Life
The Longest Journey
Beyond Good and Evil
Max Payne
Zanzarah
The Myst series
Red Alert 2
Starcraft
Darwinia
Pharaoh
...I do have trouble sorting my list of games overall in order of enjoyment, but I think that's the top of it.

Wizardy One - Proving Grounds
Bards Tale Two - Destiny Knight
Might and Magic VI - Mandate of Heaven
Doom
Eye of the Beholder Two - The Legend of Darkmoon
Baldur's Gate II - All
Arcanum
Divine Divinity
Witcher
Ice Wind Dale
Jade Empire
Dungeon Siege - Both One and Two
Planscape Toment
Temple of Elemental Evil
Dragon Age: Origins
... and the list goes on

UFO Enemy Unknown
Ultima (4-8)
Baldur's Gate (1+2)
Quest for Glory (except 5)
Civilization (2 and 4 in particular)
Monkey Island (esp. LeChuck's Revenge)
TIE Fighter
Diablo 1+2
NWN2 Storm of Zehir
and many more.
and of course one that I haven't mentioned in a while,
Game X.

I can't decide. I like half the games I play! :/
Quake
Killing Floor
HoMM5
KoTOR
Xcom: UFO Defense

Delixe
Make a list!

Rise of Nations Gold (I've probably spent 25% of my gaming life on this one alone!)
King's Quest series
Space Quest series
Clive Barker's Undying
Startopia
Star Control II
Sanitarium

Oh man, so many good games, picking the crème de la crème is going to be tough.
Planescape: Torment
TIE Fighter
Starcraft
Psychonauts
Baldur's Gate 2: SoA
HOMM3
Morrowind
Diablo 2
X-Com: UFO Defense
Civilization 4
VTM: Bloodlines
Prince of Persia: SoT
Beyond Good and Evil
Risen
The Witcher
Gothic
King's Bounty: The Legend/Armored Princess
Sanitarium
The Longest Journey

Quest for Glory series (including 5:)
King's Quest series
Planescape: Torment
The Longest Journey
Dreamfall
The Elder Scrolls III: Morrowind
The Elder Scrolls IV: Oblivion
Fallout 3
Mass Effect 1 & 2
Dragon Age: Origins
Indigo Prophecy
Heavy Rain
The Witcher
Culpa Innata
Titanic: Adventure Out of Time
The Journeyman Project 3: Legacy of Time
Laura Bow in the Dagger of Amon Ra
The Legend of Zelda: A Link to the Past
Myst
Uncharted 2: Among Thieves
& I'm now able to play so many games here on GOG that I was never able to before!

KotOR1&2
Deus Ex
Planescape: Torment
Total Annihilation+Core Contingency
Team Fortress 2
Half Life1&2
Starcraft
Unreal Tournament
System Shock1&2
Mass Effect 1
Im probably forgetting a ton.

My favourites:
Orange Box
Prince of Persia: Classcis
Prince of Persia: Sands of Time + Warrior Within
Beyond Good And Evil
Loom
Max Payne 1+2
Braid
Cold Fear
Resident Evil 1-4
Psychonauts
STALKER: Shadow of Chernobyl
Second Sight
Sniper Elite
Tomb Raider: Anniversary +Legend
Zeno Clash
Splinter Cell: Chaos Theory
System Shock 2
Batman: Arkham Asylum
Postal 2
MDK 1+2
Sanitarium
Longest Journey
Silent Hill 1-4
Condemned
PENUMBRA
Call of Cthulhu
Dead Space
American Mcgee's Alice
Metal Gear Solid 2: Sons of Liberty
Devil May Cry 3
The Saboteur
I find many posts here really one-sided, because Console-games are being left out.
Let me help you with that;
Yoshi's Island
Pocky and Rocky 1+2
Tales of Phantasia
Donkey Kong Country
Donkey Kong Country 2
Contra III: The Alien Wars
Earthbound
E.V.O. Search for Eden
FF IV-IX
Soul Blazer
Illusion of Gaia
TERRANIGMA
Legend of Zelda:Link to the Past + Ocarina of Time
Parodius
Prince of Persia(Snes-port)
Secret of Mana
Secret of Evermore
Super Metroid
Banjo-Kazooie
Blast Corps
Conker's Bad Fur Day
Mischief Makers
Perfect Dark
Sin & Punishment
Turok 64
Alundra
Crash Bandicoot 1+3
Einh�nder
Parasite Eve 1+2
Ghost in the Shell PSX
Jersey Devil
Jumping Flash 1+2
Metal Gear Solid 1+2+3
SPYRO
Vagrant Story
God of War 1+2
Kingdom Hearts
Devil May Cry
ICO
Shadow of Colossus
Okami
Shadow Hearts
Timesplitters: Future Perfect
Haunting Gorund
Most are only old consoles, but better than any shit you see today.

Civilization IV
Gothic 2
Witcher
The Longest Journey
Planescape Torment
Baldur's Gate 2
Space Rangers 2
King of Dragon Pass
Medieval II: Total War
System Shock 2
Knights of the Old Republic
Jagged Alliance 2
Fallout 2
Deus Ex
Crusader Kings
King's Bounty: The Legend
Plus all those great(est) games I forgot.

Mount & Blade (with Warband)
Team Fortress 2
Unreal Tournament 2004
The Witcher
Duke Nukem 3D
Gothic 2
Diablo 2
Torment
Shock 2
Deus Ex
Company of Heroes
Guild Wars
Baldur's Gate 2
Sanitarium
In no order.

Just a handful of some of my favourites over the years:
Monty on the Run
Jet Set Willy
Elite
The Bards Tale
Streets of Rage 2
Phantasy Star
Laser Squad
Eye of the Beholder 1 and 2
Mirror's Edge

Knights of the Old Republic
Halo
Might and Magic IV
Prisoner of Ice (GOG should really try to get this one)
Monkey Island
Zork
Enchanter
Master of Magic
ADOM
Theif