Counter argument for MOST deserving.... While Lupin had direct experience with the Dark Arts, given being a werewolf, I want to put in an advocacy for Severus Snape. He was knowledgeable, having worked on both sides of the war .. and was able to fool Voldemort into trusting him. Like, if you can fool one of the most evil wizards of the time!?! Not to mention he did actually in his bizarre way, care about the well-being of the students and their education. He had this sort of no-nonsense policy that the position needed. And finally, he WANTED the position. Being in a position that you actually want and are skilled enough to do, is a rare find and when you are in a position you want, it's never actually work.

As far as least deserving, I kind want to say Umbridge here because she was just bloody awful. Not to mention that she genuinely despised the students and didn't want to really be there =\ goes back to about Snape being most deserving because he wanted it, Umbridge was the exact opposite