The ultimate choise is yours. You just have to llok at your stats. If you have the original Besaid Aurochs team, then Keeppa would have to be the goalkeeper (appreopriate name, isn't it?), Jash in CC position, as he's both good at defending and starting the attack. Tidus is definitely a forward, so pu him one the right or on the left. I have played with him on the left (as it was default), but left defenders from opposite team attacked him hard, to me they seemed harder than left ones, so I changed sides and scored a lot more that way. You can get an excellent team, the most valuable players are Tidus, Jash and Keeppa. I have scored some 65 goals during a league, all by Tidus, and Keeppa hasn't missed a single ball.
Your defenders must have high ATTACK points (so they can attack opposite forwards), your forwards high DEFENCE points (so they can withstand opposite defenders attacks), middle fielder must have moderate, ATTACK, DEFENCE and PASS points to perform all kind of actions. Hope it helps a bit.