I think GTA 3 has this award by far. Simply because GT3 is just a rehash of older games, unlike GTA 3 which feels like a completly new game. Infact not only does it feel different to GTA 1 and GTA 2 but it feels different to all the other games out there, its a unique expirence.
This unique exprience may have slowdown and not the best graphics on the PS2 but you still feel like your in the game. The level of detail is amazing. I prefer detail level anyday to graphics, although you know me... I HATE slowdown!
Slowdown aside however GTA 3 in my opinion is in a different league to the likes of Onimusha Warlords, Red Faction, GT3 etc.
I am a PAL gamer though so I haven\'t expirenced the likes of Metal Gear Solid 2, Tony Hawks 3, Silent Hill 2 (both THPS3 and SH2 have just been released) etc etc.
Simply put, GTA 3 is long lasting, fun, got tons of replayability and is unique. Attributes which have been missing from games for a long time (games being completed in 10 hours, games being rehashes of others etc).