That would be as distinct from the mentally unstable original.When she made out with the Doctor's mentally unstable clone right in front of him?The Doctor and Rose (they never got it together and the Doctor is generally asexual but they had great chemistry anf their last scenes together had great poignancy)

Rose and the Doctor (particularly Nine,imo) have a great relationship, though.
My choices include some comic book ones:
Scott Summers / Jean Grey from Chris Claremont's run on Uncanny X-Men and Bruce Banner / Betty Ross from Peter David's run on The Incredible Hulk.