Definitely Squall- the guy is just such a dynamic, life-like character.

Also, I think Tina can surely be called the main character of FF VI. She alone is on the game's Japanese box and cartridge (right?), and there would be no story without her. She's the first character you get, she provides the link between the humans and espers, and the emotional crux of the end of the game is all Tina-centered. Think of how lame the story would be without Tina. Some of the other characters are pretty important too, but not so utterly essential to the story.