1. Tidus (X)
2. Cloud (VII)
3. Squall (VIII)
4. Zidane (IX)
5. Lightning (XIII)
6. Serah (XIII-2)
7. Yuna (X-2)
8. Bartz (V)
9. Cecil (IV)
10. Vaan (XII)

Now why did I put Tidus at #1? Simply because he's EXACTLY the protagonist FFX needs. I left out VI because I don't feel there is a definitive main protagonist in that game. I'd have almost left out Lightning as well, but she's enough of a main protagonist to qualify.