Well there is no a "Best" for the whole series, but there is one or two for every game.
FF1 : none ofcourse
FF2 : Leon was kinda ok, but nothing spectacular.
FF3 : None ofcourse.
FF4 : Kain was the one who really got the best design, and the ending left an opened door for his fate. (My fav FF4 character though is Edge)
FF5 : Gilgamesh.
FF6 : That was hard, but I think Setzer had a one of a million spirit really . Shadow is a close second (While I really wonder why would he leave Relm without telling her the truth)
FF7 : Sephiroth, Cloud or Vincent. Ok I think it was Cloud since Square really made effort of showing how a very normal person with no special talents can be the hero of the game.
FF8 : I couldn't just find a design in FF8 that I can wonder how much time was consumed in giving him all this background. But after some thinking I couldn'tthink of FF8 without Seifer (Anyway he cut Odin to two pieces ). But that doesn't mean I liked Seifer.
FF9 : Kuja and Garland were two unique designs. But I'll say Kuja is better since Garland's personality kinda resembled Fusoya's in FF4 (with the difference in intentions though) Anyway I think Kuja was mistakenly considered a male, mybe it was a translation error (j/k , unfortunatly it was on purpose to make him a gay male . Why you ask? To compete with the Ape man , /looks at the hero)
FF10 : Auron no doubt. One of the best ever designs in FF world.
So what's the best of them all in terms of design? It'll be either Auron or Sephiroth.
lol... no no you got me wrong. I have no problems with her personality she is a great person to be with/hang out with. I was just saying it would be cool if she would get a little crazy sometimes... she is always calm and controlled. But really i shouldnt even be complaining. /me slaps himself