This question was... insanely difficult to answer. There are SO many of them that I like and the points brought up by Jess and Brigette made sense and had me second guessing which one I would choose! But in the end, I have to agree on Remus Lupin! despite probably needing a sub once a month, I still think Lupin is one of the most qualified of the Professor's we've seen and one of the best teachers. Not to mention if you're having an off day, he'd probably give you chocolate. So really.. how is there another choice?
Though I will also admit, I would be VERY curious to see what Mad Eye Moody would have been like in an actual teaching position. No doubt he was very skilled and would have had a lot of great information to give students given his first hand experience with DADA, but I would have loved to see his personality with the students as well!
Jess, I'm curious what is it about Professor Vector that also draws you to them? :3