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