The deathblow sound effects are pretty satisfying in Dragon Age Origins... Those decapitations are nasty.
A Zonda going through a tunnel in Forza 4's pretty incredible, ditto in TDU with the windows down.
But surely (if you played it in the mid '90's and were a teenage boy) Lara Croft's soundscape of grunts, sighs, and moans are the defining soundtrack to a game!
Oh, Gouki's triumphant "MESSATSU!" ain't bad, either (especially with the accompanying Super/Ultra Finish 'chime'). Street Fighter IV's SFX are, like the rest of the game, nigh on perfect.