FFX dosent have a "leader", if your thinking of a leader is the whole quiet/anti social/forgets his past guy. Like FTF said, all the charicters are important. But if I had to list wich ones made the best leaders, iw woild be:

1 Auron
2 Lulu
3 Tidus
4 Yuna

and so on. But meh, just my opinion.