If Professor Slughorn could not be in Slytherin (which, I'm sorry, is SO his house) then I would have to place him in Ravenclaw. Part of me wants to try and sort him Gryffindor just to be contrary, but I really can't bullshit enough reasons to justify it. But as for Ravenclaw, he is obviously very intelligent as illustrated by his proficiency at potions, and he seems to display an aptitude for thinking outside of the box, a trait which I associate with the eagles. He thought to transfigure himself into an armchair when Dumbledore came looking for him. Most people would cast a spell to simply make themselves invisible. He went for furniture. He also is considered an excellent professor, which requires you to be able to look at things from many views to get different people to learn the same material. Since not all students respond to the same teaching methods, you need to be able to switch things up and try new things.

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