Personally, a Centre Back or a central midfielder. Without a good player in that position, your going to struggle. Build from the back, build a good defence, make yourself hard to beat and then worry about your attack.
I like to build a solid defense, and go from there. That means a couple solid CB's and preferably a solid defensively oriented CM (can be DM or CM with defensive/ball winning duties).
Generally that's how I like to build all my teams. Create a solid defense -> build a goal scoring machine on the basis of that.