I don't think anyone's going to deny that Harry/Ginny, and really Rowling romance writing in general, was a bit weak.

Personally, I'd say that in terms of writing the two best-executed relationships were Harry/Cho and Ron/Lavender. Troubled teenage relationships that were never going to work out are probably a lot easier to write well than the sort of high school true love that that would be expected for the main pairings.