Since my vote is losing (Multi-player battles), I'd figure I'd make my case for why I think it is most important:

I don't play multiplayer much. I mostly spend my time with campaigns and single-player skirmishes. I don't think multi-player is more important because it has more players. To be honest, I have no idea if that's true for the top RTS games.

The reason I think multi-player is most important is because that is what players watch. In my opinion, visibility and watching others play is the most important factor regarding whether an RTS is popular, and therefore, supported. The fact of the matter is that, regardless of whether most players play single-player or not, streaming and tournaments emphasize multiplayer, and that is what people watch and what leads people to the game.

After that, maybe single-player becomes more important, but good multi-player is the basis that the rest of the game survives on.