Old stuff
King of Rock - Run DMC
Humpty Dance - Digital Underground
Check the Rhime - Tribe Called Quest
If It Ain't Ruff - NWA
My Posse's On Broadway - Sir Mixalot
A Bedtime Story - Slick Rick
Poison - Bell Biv Devoe (gotta love GTA-SA soundtrack)

New stuff
Picture Me Rollin, Heartz of Men - Tupac
Nasty Boy, Sky's the Limit - Biggie
Come to think of it, just about any Tupac or Biggie (won't get into the immortal "who's better?" argument)
I am #1 - Nelly
Disco Inferno - 50 Cent
Bells of War - Wu-Tang

EDIT: Where the Player's Play remix feat. Nelly and Snoop...don't know who the guys from Atlanta and NY are, but that "is the joint" - as my friend Ed would say.

ScottieK