Hagane no Renkinjutsushi
Also known as Fullmetal Alchemist, Hagane no Renkinjutsushi's pairing of Roy/Ed
has such a rabid and broad fanbase, it's hard to believe these two 
aren't canon.
I originally ran across the pairing of Roy Mustang and
Edward Elric
while looking for fic of the decidedly more canon
Roy/Riza variety.
I read one fic, then another and another, and before I
knew it I had crossed over to the dark side forever. For most
people, the major squick factor with this pairing would be the fact
that Roy is a grown man and Edward is barely into his teens. Being
the depraved shota-whore that I am, I find this to be the extra
spice that makes this pair irresistable. There are many folks out there who have managed to capture in fic form the essence of
these two: their strong male egos, their bickering, Ed's obstinate nature and Roy's
smugness and sarcasm. I wish I were one of them. *sigh*