Who to choose, who to choose!
I say threesome. Got one hot/cute man, one hot chick, and one cute chick, and you have it made! Just kidding!
Anyway, from what little of the original game I've played, what I could understand of Advent Children, and any research I have done. I can't see any of them together.
I mean, when Aerith was alive, I could see her as the mischievous, little sister that Big Brother wants to protect. But, in AC, I came to see Aerith as more of maternal figure to Cloud and the others (although her mischievous attitude was still there).
As for Tifa, I can see something between her and Cloud. In FFVII, Cloud mentions her as being his first crush, and wanting to impress her. He also makes a promise with her. And, I can see Tifa crushing on him in the original game. However, Cloud's feelings were a little confusing to me, even after Aerith died.
There is definitely something deep between Tifa and Cloud, but I don't think it is a mutual relationship. Cloud just seems too shy and withdrawn to make his feelings known, and Tifa is too independent and tomboyish to admit her feelings (she could also be a romantic, waiting for her knight in shining armor to confess his love to her and sweep her off her feet).
It is possible for a man and a woman to live together, and love each other beyond that of friends, family, or lovers.
EDIT: Besides I see Aerith more with Zack than with Cloud.