My team dominated the blitzball turf, going undefeated for the enite game (meaning the 50-odd games I played to win that blasted Jupiter Sigil). It was composed of:

Right Forward: Tidus
Left Forward: Wakka
Midfield: Brother
Left Defense: Jassu
Right Defense: Letty
Goalie: Miyu

Tidus was a great scorer from the start, since his Jecht Shot took most defenders out, and later in the game his Sphere Shot put his shooting stat over 40, and no goalie could touch it. I used him almost exclusively for shooting, however, since his passing sucked. Tackle slip helped him out a bit.

Wakka was also a good shooter, and again served mostly for that purpose with Wither Shot and Spin Ball, because his passing and defense are terrible.

Brother completely dominated the field as center, leveling up past everyone until his level was in the 50s by the time I quit playing. He was so fast no one could touch him, had enough defense and passing points to survive hits by at least 3 opposing team members, completely knocked opponants out with one tackle, and stole the ball almost every time it was tossed his way. With anti-wither and venom he was pretty safe, and tackle slip avioded him almost all impediments. It was wicked.

Letty and Jassu were about the same. Both had really good passing numbers, and their endurance was good. They also had plenty of passes and level 3 tackles, and tackle slip helped them out.

Miyu was an excellent goalie from the start - almost nothing got by her, especially after learning grip gloves and super goalie. She also came with anti-almost everything, which made her a powerhouse.