The only game ive played solidly for completion without other games gettin in the way, was Gran Turismo 2. Borrowed it off a mate, 28 days later i handed it back having fully completed both discs.
Racing - Gran Turismo 2 ( if that wasnt already obvious

)
Fighting - close battle between Street Fighter Zero 3 (PSX) or DOA 2 (Dreamcast)
FPS - CoD4 ( for the modern stuff ) Unreal Tournament 2004 ( for pure arcadey joyness )
Flight Sim - Until i get HAWX, Star Trek Invasion (PSX)
Sand-Box - GTA: San Andreas
RTS - CnC3: Kane's Wrath ( altho that might change as ive just got Endwar )
RPG - Fable: Lost Chapters