Ginny and Dean: Even though he made Ginny cry a lot, I think Dean and Ginny made a sweet couple. Their involvement in the D.A. shows they share a common cause. Ginny is known to be a passionate defender of the underdog (see: Luna Lovegood), and Dean is the same, fiercely defending Remus Lupin when Umbridge called him a “dangerous half-breed.” Of course, Ginny had to end up with Harry, so the relationship ended, but I’m sure she was good for him while they lasted.
Percy and Penelope: I think Percy and Penelope were the ideal couple. Both prefects, both passionate about their House winning Quidditch matches – they even make a bet on a Gryffindor/Ravenclaw game – so they’re clearly one of those adorable competitive couples. Percy’s very driven and intelligent personality would definitely match well with a Ravenclaw.
Viktor and Hermione: The surly international Quidditch star really had a soft spot for Hermione. While he was introduced as a brooding and famous athlete, around Hermione he was tender and kind, enjoying just hanging out with her while she studied. It’s evidence they had a good relationship, as their friendship lasted for years, exchanging letters even after the Triwizard Tournament.
Cedric and Cho: Oh, tragedy. Cho was such a happy girl with Cedric. The two Quidditch players were both such sweet characters and loyal friends and shared a genuine belief in and care for Harry. Cho and Cedric would be that couple that everyone felt invested in even if they didn’t know them; they’re just too adorable.
Remus and Tonks: Even though they did get married, their relationship was plagued with issues and ended only a year after it began when they were both killed. I believe a Hufflepuff and a Gryffindor make an ideal pair, with a mutual passion for doing the right thing and putting their lives on the line. After losing his friends, Remus needed someone who would accept and care for him the way Tonks did. She also never put up with his crap, as revealed on Pottermore: “You’d know perfectly well who I’ve fallen for, if you weren’t too busy feeling sorry for yourself to notice.”