Mains, huh? I tend to use (or attempt to use) lots of characters, but I'll list my "go-to" ones.
SSB - Captain Falcon, Samus, Fox
SSBM - Captain Falcon, Samus, Mario
SSBB - Captain Falcon, Zero Suit Samus, Snake
KOF98 - Lucky/Shingo, Ralf, Clark
KOF02 - K'/Andy, Ralf, Clark
KOF03 - Ash, Jhun, Clark (L)
KOFXI - Maxima, Shen/Terry, Clark (L)
3s - Q, Ken, Makoto
ST - Guile, Dee Jay
SFA3 - Cody, Charlie
MvC2 - Cable, Guile, Captain Commando
MSHVSF - Spider-Man, Wolverine/Akuma
GGXXAC - Order Sol, Slayer
CVS2 - Terry, Yamazaki, Geese (K-Groove)
NGBC - K', Haohmaru
SVC - Geese, Guile, Chun-Li
Garou - Terry, Gato, The Griffon
SCII - Raphael
T5 - Asuka, Bryan