You don't really need 99,999 HP if you've got 255 on magic defence and normal defence because if something does more than 9,999 and you have those 2 stats then it'll kill you no matter what and you'll need Auto-life.

My best characters are Wakka, Tidus and Yuna. Wakka is amazing with his limit break Attack Reels (12x99,999). FFX is strange because the only real difference between the characters are their limit breaks but if you have Wakka in your team and the Entrust magic, it doesn't really matter who else you have. You can sacrifice Yuna's Aeons in some of the harder battles agains like the Dark Aeons and Penanze(probably spelt wrong there!).