Tommy Vercetti all the way and I guess after that it would go Niko, Claude, CJ, Vic and Tony (never played LCS though). Hopefully Johnny will be a good character and not just some typical biker.
As for NPCs...
GTAIII: Donald Love (haha fuckin' hilarious)
VC: Love Fist and Phil Cassidy
SA: The Truth
LCS: n/a
VCS: Martinez (I guess, like Bear said they sucked)
IV: Packie, Roman, Little Jacob