PS1:
FFVI
FFVII
Wipeout

PS2:
FFX
FFXII
GTA:SA

Gamecube:
Pikmin
Viewtiful Joe
Zelda:Windwaker

Xbox:
Knights of The Old Republic 1
KOTOR2
Fable

PS3(at the moment):
Resistance:FoM
The Darkness
Ninja Gaiden Sigma

1) GTA: Vice city / Goldeneye64
2) FFX
3) GT3

Biggest let down ever is FF12.

Top 5

The Legend of Zelda: Ocarina of Time
The Legend of Zelda: Link to the Past
Super Mario Bros. 3
Resident Evil 4
Super Mario 64

My would be:

1 - Suikoden II (PS1)
2 - Final Fantasy VI (SNES)
3 - Metal Gear Solid (PS1)

Suikoden II for me was the best story in an RPG ever. It felt so epic how this two best friends who wanted the same thing but went in completely different ways and the ending (if you get all 108 stars) is amazing. The song in the opening is also awesome.

Final Fantasy VI was the best Final Fanatasy for me. Since IX, I dont know what happened, but I really dont like the story in the Final Fantasy and XII was the first one I didnt finish because I didnt care anymore.

Metal Gear Solid was a game that I didnt know when I bought. I only bought it because I really wanted to buy a game so I wouldnt go home empty-handed and it turned out to be a great game with really cool bosses and history.

Honorable mention - GTA III. A game that was revolutionary at the time and provided countless hours of fun 

(and the also great sequels Vice City and San Andreas).  

1.Xenogears (PS1)
2.Super Mario RPG (SNES)
3.Sonic the Hedgehog 2(GEN)

ps2:
KH
Kh2
God of war

GC:
Paper mario 2
SM sunshine
Pokemon XD

n64:
Mario64
Banjokazooie
Banjotooie

Ps3
Heavenly sword demo
resistance fall of man
sadly...spiderman 3

1. Super Mario World (Best game ever. No game has yet to top it)

2. Gears of War

3. three-way tie: Resident evil 4, Tactics Ogre: The Knight of Lodis, Riviera: The Promised Land

Resident Evil 4
Super Mario Bros 3
Chrono Trigger

I actually didn't have to really think about it that much. These are games I have played over and over again and never get tired of. The only thing I go back and forth on is which order to put them in. That's my current order.

1. God Of War 2 (PS2)

2. GTA San Andreas (PS2)

3. Wii Sports (Wii)

Final Fantasy 7

Perfect Dark

Hmm... Maybe Super Mario World.

I hate the best tag, these are probably my favourite games, far more accurate description.