each game seems to have one/two characters that people love to death
I: Fighter/Black Mage
II: Lionheart
III: Onion Knight is very well loved
IV: Kain as so many fanboys, and then Rydia
V: Galuf or Faris
VI: This one is tough because there are so many characters, personally it's Cyan, but the most popular seem to be the two main, Terra/Locke
VII: Sephiroth by far, but then I'd say the Turks
VIII: This one I'm not sure of. People either love or hate characters in this game. On that I'd say Irvine/Zell.
IX: Vivi, obviously
X: Auron

NOTE: This is just my observation of what I see most people say.

THE JACKEL