I'm honestly shocked that Squall is doing so well. He was the weakest main character in the series from FFIV on if you ask me.

Terra gets my vote with honourable mention to Tidus and Zidane. I found all of these characters more likeable than the others, and better developed. In fact, although Tidus isn't getting the nod from me, I've got to give him credit for being one of the most believably developed characters in the series. Nothing about the changes you see in him over the course of the game felt forced or unnatural to me.