The manager.

Defensive midfield imo is the most crucial position on the pitch, without a great one or two you can't win nothing just ask Arsenal who have not had one since Viera.

What do you consider to be the single most important position in a team?

I think its the Fullback. A modern fullback has got to be so fit, he's a winger, and he's a defender. A great fullback scores a lot of goals, but stops more.