Snape/Filch! I'm actually very fond of this pairing. ;-)
I love how subtly you portray the transitions in their relationship: starting off as colleagues with a mutual interest in punishing "the grubby lot", then becoming companions, then becoming... not exactly lovers; there's nothing sweet or romantic about these two... partners, perhaps? And then, it all falls apart due to Filch's pride and Snape's lack of understanding. So very realistic -- and sad.