San Andreas, cause it's a fictional state! There's three major cities, a few small communities, and countryside to explore. How can Liberty City and Vice City even compare?
The gameplay in SA was also a major step up. You could swim. Fly jets, parachute, use a jet pack, use stealth techniques (not the best, but it was there), and much more.
GTA:SA is arguably the best GTA game out there right now.