Tidus and Yuna is the obvious answer, I thought they had good chemistry in the first game. Their love story in FFX is basically the only one I have ever liked in a FF before because it wasn't so forced, hollow, pre-established or insignificant. X-2 ruined it when Yunas personality was slaughtered to that of a slutty, giddy popstar.

I could never see Lulu and Tidus together, Tidus would be far to much of an energetic child for Lulu. Wakka was silly but he was not really a bottle full of nagging energy that Tidus was.

If someone gave me personality descriptions of all these characters on paper without including situations and events that happened I could definitely see Tidus and Rikku hitting it off. Besides coming from different worlds they actually have almost everything in common when it comes to their personalities. But perfect matches don't mean everything because that doesn't mean that's exactly what the other person needs or wants.