I like the Tifa/Cloud thing (in the main game) and how they incorporated Aeris as well to make it an awkward triangle. The way they twisted that with the Zack-Cloud and Zack-Aeris stuff made it even better.

I like the Serah-Snow thing in XIII but I feel they screwed it up in the two sequels. I feel SE felt they had to water it down to ensure that people could feel there is a "thing" between Noel and Serah, and it ruined it all. In XIII, Snow would show his love for Serah at every opportunity. In the sequels, this wasn't as much the case and I didn't like it how it panned out.

Locke and his past lady was a nice romantic twist to his overall story.