Keep Tidus, especially if you have Jecht Shot. Because it knocks away 2 defenders, Tidus' low EN stat does not really matter. And his high SH stat means that his shooting can still go through the remaining defenders and still be strong enough to beat the goalie.

Brother is another good option. IMO he is extremely fast. IMO he makes playing Blitzball rather too easy.

Nimrook is a great keeper. Sign him up immediately after he leaves the Al Bheds. In the meantime you might want to keep Keepa or Durren as your goalie. But none of Durren's stats exceed 30 (except for SP), so he gets pretty useless later on.

Wedge is good too. Wedge has good SH, but later he has more AT, making him better for defending. After that (Level 70+), his CA increases so much that he could be good enough to be a goalie.

Wakka sort of sucks. His shooting is average, but his defense is awful. He's only useful if you use the Auroch Spirit ability, but then again, you may not have any Besaid Aurochs in your team, making it useless.