I agree that Slughorn is a Slytherin at heart, but if he were to be put into another house, I think I might actually go with Gryffindor. Although he's undoubtedly smart, we don't seem him focused a lot on learning for learning's sake; there's also nothing to suggest that he's particularly hard-working or loyal.* First, although he is not overly courageous, he's certainly not a snivelling sniffpot, either. He eventually gave Harry his memory, even though he was deeply ashamed of it---something that takes a great deal of courage, especially for someone who is very concerned about his image and connections; additionally, he fought at the Battle of Hogwarts. Second, he seems to have a real passion for experience, which I've always felt is a very Gryffindor taste. Sure, a lot of his favorite experiences seem to involve comfort. But he seems to genuinely enjoy and take pleasure in lived experience, whether it's eating crystallized pineapple or going to Harpies matches.

Anie//Ravenclaw

*Since he's a teacher, he's obviously not a duffer, but I have a sneaking suspicion that getting a teaching job in the wizarding world is a lot more about knowing the Headmaster and being competent, and a lot less about academic brilliance and toil.