I don't think I could narrow it down to just one, but I love cajun cookin' and guess may cayenne (red) pepper would be at or near the top. In fact, this is interesting timing, because I just sat down here after mixing my own dry rub to put on a brisket tonight:
2 parts Garlic Powder
2 parts Paprika
1 part Cayenne
3 parts Salt
3 parts Brown Sugar
1 part Cumin
1 part Chili Powder