West:
2pac - To live and Die in LA (maybe Cali Love, but Live and Die has that laid back westcoast feel)
Snoop & Dre - Nuttin but a G thang (classic shit)
East:
Biggie & Jay-Z - Brooklyn's Finest (or The What with Method Man)
Nas - Represent (Primo at his best)
South:
Outkast - Player's Ball (blew up Outkast, shit came out on a Christmas album, damn)
Lil Jon, Usher, Ludacris - Yeah (Recent song, but made Lil Jon and Usher household names)