Mine are, in no particular order.

Baldur's Gate II
Final Fantasy VI
The Last of Us
Portal 2
Secret of Mana

Whatever I'm playing, seriously. When I get really into a game all I keep thinking when I'm playing is, "This is the greatest game ever!"

A few weeks go by, I get into another title, and of course I'm thinking, "Scratch that, THIS is the greatest game ever!"

Clearly you haven't played Dragon Age: Origins or Culpa Innata :)

From most to least favourite:

Albion
Dune 1
Baldur's Gate 2
Galactic Civilizations 2
Sid Meier's Alpha Centauri / Alien Crossfire
Master of Magic
Neverwinter Nights 2
Starcraft 1
Master of Orion 2
System Shock 1 and 2


Games I can't rate, because they are too good / left too big an impression on me:

Planescape: Torment
The Cat Lady

Vampire:The Masquerade - Bloodlines
Baldur's Gate 2
Mirror's Edge
Ys Origin
The Cat Lady
XCOM Enemy Unknown

Fallout 2 and Chrono Trigger. These are games that I can keep on playing over and over again despite the fact that I have pretty much mastered these games.

No order and limit of 20 (don't want list to keep going :P)

PC wise,

Albion
The Elder Scrolls series
Treasure Adventure Game
Driftmoon
Stonekeep
Lands of Lore series
Everquest
Might and Magic/Heroes of Might and Magic series
Creatures series
Unreal/Unreal Tournament series
Starsiege: Tribes
Worms 2/Worms Armageddon
Hover
Brix
Ancients 1: Death Watch/Ancients 2: Approaching Evil
Rodent's Revenge
Chip's Challenge
Grim Fandango
Callahan's Crosstime Saloon
Duke Nukem 3D: Atomic Edition

* NWN 1/2.
* GTA VC.
* Quake 3 Arena.
* UT.
* Starcraft.

I have not tried, Baldur's Gate yet, I bought a copy, its in list of games I wanna play. Currently playing PST.

That's a tough one. But Planescape: Torment and VTM: Bloodlines are definitely among the all-time-favorites, as well as most of the LucasArts adventures, specifically Monkey Island 2, Day of the Tentacle and Sam & Max Hit the Road. And I love Forgotten Realms Unlimited Adventures and Neverwinter Nights for their community content.

Apart from that, I guess I always have a few current favorites each year; so far, in 2013 I had a lot of fun with Enclave, Blade of Darkness, Dungeon Siege 2, Hotline Miami and Waking Mars.

Civilization 4
Baldur's Gate 2
Baldur's Gate 1
System Shock 2
Morrowind

Been awhile but I remember liking
IWD 1 & 2
Thief 1-3
Civ 3
Half-Life 1 & 2
Rainbow 6
Starlancer
Age of Empires 1-3

Baulders Gate 2
Planescape Torment
Outcast
Flat Out: Ultimate Carnage
GTA 3 / VC / SA
Burnout Paradise and Need for Speed: Most Wanted
Wing Commander series
Soul Reaver
Anachronox
Half Life
Deus Ex
Portal
and back in my Amiga 600 days I used to play the original Civilisation until 6am in the summer holidays!

Driver and Interstate 76 aren't too far off either (not Interstate 82 though, never that - eugh!)

Planescape: Torment is my favourite. After that, in no particular order other than alphabetical:
- Baldur's Gate
- Chrono Trigger
- Fallout
- F-Zero GX
- Neverwinter Nights 2: Mask of the Betrayer (if you count that as a separate game, NWN2 if you don't)
- Paper Mario
- Portal
- Portal 2
- Spec Ops: The Line
- The Elder Scrolls III: Morrowind
- The Legend of Zelda: Majora's Mask
- The Witcher 2
- Vampire: Bloodlines

And some other shit I forgot.

In no particular order
Baldur's Gate 1&2
Planescape Torment
Fallout 1&2
Master of Orion 2
Imperium Galactica 2
Civilization 3
Can't decide :)

Top 10:

1. Planescape: Torment
2. Fallout 2
3. The Longest Journey
4. Thief
5. Syndicate
6. Full Throttle
7. Fallout
8. Goldeneye
9. The Legend of Zelda: Ocarina of Time
10. System Shock 2