Jecht shot 2 - it took me so long to get it but it was very disappointing when I got it. You need Tidus's KEY TECHNIQUES (they are Venom Tackle, Drain Tackle 2, Anti-Venom 2). Once you have all these techniques you can win the Jecht shot 2 as a prize in the Blitzball tournaments. If you don't have the key techniques you will not be allowed to learn Jecht Shot 2.
It uses 999HP, knocks out 3 opposing players, I think it also has Invisible shot in it but I'm not sure. I don't think it is worth getting.