Pun...Oh ... For me the Title question is a little different from the description. If i would choose one CPU, i would get a FB, you can just use run plays without FB and have good running game, trips, SB, spread...but would i call it the most useless position? Nope, it is very usefull.
Though any 3 of those could be somewhat valuable in a certain system, in many cases any 3 of those could be replaced with a cpu and do not much harm to your overall plan.
If it's the only player at that position then I'd say FB. If you are having multiple players at the position then I'd say Center and put the CPU on special teams only.