Second favorite: Harry/Hermione
Others: Harry/Luna, Remus/Sirius, Lucius/Narcissa
As you can see, my favorite pairings aren't canonical except for Lucius/Narcissa. That's mainly because I don't find the couples in canon that interesting or even well-written, some were just placed there completely out of left field while others I just didn't agree with. I love JKR for creating this world and all, but I don't think romance is her strongest suit in writing, imo.
Rest of the 30 Days of Harry Potter Challenge.