I've always thought that Snape/Hermione was regrettably the biggest het pairing in the HP fandom

I doubt that, but on the other hand, there are no hard numbers. I've certainly been left with the impression that Ron/Hermione is the number one het pairing, with Harry/Hermione trailing. It seems to us that Snape/Hermione is very popular since we're both in Snape fandom (and neither of us like the pairing), but I don't want to make the mistake of taking our corner of fandom as representative of the whole, you know?