Definitely the Doctor and Seven. They had a Henry Higgins/Eliza Doolittle (My Fair Lady) relationship already, so a romantic attachment would have worked nicely. However, Seven tended to see the Doctor as a father-figure, so that was more true to her character.
The Seven/Chakotay nonsense in "Endgame" would have worked if it had been rewritten to be Seven having dates with holo-Chakotay, or real-Chakotay discovering her dates with his holographic self.
Tasha/Data. Drunk or not, it did
happen. And it would have done so much more to develop Tasha's character so maybe the actress would not have left (she was my favorite TNG character).