i would have to say starcraft and i wouldn't class the civ series as an RTS its more a Turn based game. like Total War.
I voted for Warcraft. For me personally, that is my favorite of these 4 games. I never got into Starcraft, so I can't say if it is better than Warcraft though, but I have spent many days and hours playing Warcraft, and I would play it again to this day.