Out of the four, I would say Squall and Rinoa. Now anyone that has read a few of my posts knows FF8 to be my least favorite, but the fact is that it's the most likely to really happen. Plus, Squall becomes her blade, her life protector, and eventually her liberator after she turns into Ultimecia and forgets her past.

Tidus and Yuna does not seem like real love at all, most like High School lust.

Tifa and Cloud could never happen with Cloud chasing after the ghost of Aerith in his dreams all the time.

Zidane and Dagger is beastiality, after all genome does not = human. Also, this form of love is unrealistic due to the whole Queen-Thief thing, it wouldn't really work out.

I would still say Vaan+Penelo is my favorite relationship, though.