From What I heard of from my coach during school times, a captain should be calm, & compose when under pressure, they need to strategically sound because a capt. need to organise team & gives out instructions.

Therefore the BEST position is central defender or midfielder as this position gives them opportunity to analyst the game from the back. Basically it offers them a chance to view the whole game & gives out instruction accordingly.

U may argued that goalkeeper & striker also position centrally, but a goalkeeper involvement in built up is very limited in the sense that he cant gives out instructions to outfield player efficiently & is confined to penalty box therefore limited his movement.

In the other hand, a striker is not viable as he plays further up the field, therefore comminication problems arose. Also it is difficult for striker to organise play & make space for himself in scoring position.

As a central defender, he can analyst the movement of team mates & opponents. Also he needs to organise the defence & midfielders only as usually striker wont need much organisation in terms of defending and built up play. Most of the teams in the old days used a centreback as captain.

This leaves us central midfield. As we all know this is the most crucial part in modern football, heart of the team if u like. It is a position that links defence & attacks, hence there'll be no much breakdown in communication. Same as central defender, he can analyst & built up play efficiently.

All said nowadays a capt. can be in any position. I'm from old school of thought so it will be central defender or midfielder for me.