Wow Cobra, our lists could not be more different.
PSX: Too many games for me to decide, but I'd say Metal Gear Solid wasn't it.
Saturn: Panzer Dragoon Saga? Maybe VF2.
Nintendo 64: Conker's Bad Fur Day, bar none.
Sega Dreamcast: Dead or Alive 2, Shenmue 2, or Sonic Adventure 2.
PS2: I'll give that to God of War 2.
Xbox: Splinter Cell: Chaos Theory
Gamecube: RE4 or Pikmin 2
Xbox 360: Gears of War
PS3: No clue.
Wii: Mario Galaxy.
Definitely Saturn from Virtua fighter to Virtua Fighter 2 an incredible improvemen, didn't have the pleasure to play Saga though
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Sega Dreamcast I have to give it to shenmue II and Sonic Adventure 2, for Dead or Alive 2 it had a better fighting game at launch day Soul calibur
Ps2: I don't see any major improvements here it always had nice enough graphics from MGS2 to MGS3 theres no huge improvements though they are like 5 years apart, Soul Reaver 2 was there at 2001 was good GTAs had some improvements but were all close enough, Never really played GOW though and i stoped buying games for ps2 long ago.
Xbox: Splinter Cell: Chaos Theory is indeed a great improvement from the first games, Halo 2 was also a nice jump but i think the greatest on xbox was Chronicles of Riddick maybe
Gamecube i can't really see anything since the best graphics i have seen on it were from an early title Starwars: Rouge Squadron
xbox 360: kind of early to tell PGR 4 looks like a nice jump from 3 but not big enough for being a great improvement. Halo 3, Bioshock, Mass Effect, Gow might be the obvious choices but go and the others are an year apart so its weird.
Ps3: still too early too tell too but definitely Uncharted and Unreal Tournament 3.
Wii : once again is too early but i don't see any improvements here since it had Metroid Prime 3: Corruption at launch right?