Professor Severus Snape
Reasoning: This card represents an intelligent and analytical person, one who understands problems and information quickly and is good at mental challenges. This person is a talented speaker and expects others to live up to his own rather exacting and demanding standards. Severus Snape is all of those things: his logic puzzle protecting the Stone is a good example. He can see through lies and dubious characters often when no one else does (he was the only one to suspect Quirrell, and always knows when Harry has been breaking rules).
Really the only thing about this card that doesn't wholly fit Severus is that this card usually represents someone who is just and honorable and ethical. However, while Snape can be mean and petty and is definitely NOT fair and ethical when it comes to academic dealings at Hogwarts, in the grand scheme of things, Snape is on the honorable side of good. Severus made the difficult decision to leave the dark side and serve Dumbledore, and for the most part, no matter his personal feelings, manages to take the high road and do as Dumbledore requests.
In a traditional tarot deck, this is the King of Swords.
Notes
I always enjoy drawing Snape. I had this design for his face sitting around for some time, with a different cloak style that I now dislike. So, he underwent some slight alterations, got a background, and here he is for this card.
Drawing slimy things in jars was difficult, and they're still not my favorite parts of this picture. But the shelving itself was a lot of fun, and Snape himself makes me very happy as well. :)