No argument here, Sirius is definitely fascinating. Especially since OotP where we found out that as a kid he had so much to prove to himself and the others considering his background. It certainly gives him a good motive for the way he was with Snape, the reminder of his Dark Arts loving family.

And I can't argue that Sirius loves Harry. It was rather creepy in OotP, though how he couldn't tell James and Harry apart. Creepy and very sad.