Cloud and Tifa for a number of reasons.

1. Aeris is dead. You see.
2. Cloud and Tifa were childhood friends.
3. Tifa helped Cloud through the part when he was sick and she stood by him.
4. In FFVII:AC they take care of 2 kids together and live together
5. Aeris liked Cloud because of his similarity to Zack and she liked Zack.
6. This is shown at the end of AC when they show both of them together.