I've been doing a ridiculous amount of research on this subject for the past five days.
Cloud Strife's ultimate sword is the Ultima Weapon which becomes stronger the higher the users HP is. IT has a 100 Attack stat, 110 Hit Rate and gives +24 Spirit and + 51 magic. (Tidus and Lightning and Terra, from KH, can also carry this sword.) He's defeated Bahamut single-handedly and Sephiroth quite a few times.
Squall "Leon" Lionhart ... I'll be honest. I haven't played this one yet. Give me some info. Everyone always says he's OP but I don't know why.
With Rikku, she can create a barrage of bombs and elixirs just by mixing low level items. You can defeat both FFX and FFX-2 just by using her alone. Her Ultimate form is no joke either.
Auron's attacks are brutal. I spend more time reviving others than I do healing him.
Claire "Lightning" Farron can do practically everything. She's a good medic, a great ravager, and an awesome Commando. And now she's a Goddess with Odin by her side.
But Oerba Dia Vanille is also a very powerful magic user who can also use Death and summon.
Writing this made me realize that I also need to find out who is the strongest character in each job/class. gonk Here are my biased opinions:
White Mage:
Black Mage: Lulu or Vivi
Theif:
Summoner: Yuna
Warrior:
Dark Knight:
Red Mage:
Blue Mage:
Monk: