For any fight in Vegas I have, Balrog's stage music from the old-school Street Fighter II games. Against any Russian fighter, it has to be Zangief's stage music. Ditto for any Japanese fighter with Ryu's stage music. Any ex-military man, go with Guile.
That, or I would come out to Ozzy's "Crazy Train", Led Zeppelin's "Kashmir", Judas Priest's "You've Got Another Thing Coming", Guns 'n Roses' "Welcome to the Jungle", Black Sabbath's "Iron Man", and of course...Survivor's "Eye of the Tiger"!