Tifa, the strong empty-handed and so noble-hearted fighteress
Of course, my favourite one between Tifa, Rinoa, and Yuna is Tifa Heartilly !!!
For me, she is the one who can fight for somebody as good with her fists than with her heart.
Tifa is the only character that managed to save Cloud's Mind and so she makes him stronger, able to beat Sephiroth.
It's not that I don't like Linoa and Yuna, but Linoa is too fragile. She isn't sure of herself, and she needs the others to discover herself in the game. Squaresoft uses that pictures of her to make us have emotions for her and Squall. Maybe it's working, maybe it's not. It depends on everybody. But I prefer Tifa ...
As for Yuna, I think that, in the story, she's already an adult one, and her love for Tidus and her emotions in the game are those of a very self-builted woman. Yet, she can't defend her self without hiding behind G-Forces, and by having a body-guard along.
That's why I prefer Tifa !