12. Horace Slughorn
If Lucius Malfoy respected Slughorn's competence and abilities then he must be a big deal. And the fact that Slughorn evaded so many Death Eaters by turning himself into a chair is further indication of his power. Not only is he very resourceful but he sheds light on stupid the bad guys are considering how quickly Dumbledore identifies his chair disguise. Slughorn was a man who knew deep down the difference between good and bad, and although it may have taken him longer than it should have to go back to Dumbledore, he proved vital in the end.