Personal opinion - the most deserving Professor for DADA was Remus Lupin. Maybe this comes from my bias, BUT he was really good with the students when his problem wasn't known - and who better to teach DADA when he'd been personally affected by Dark Arts with his 'furry little problem' (I'm at least assuming this is considered Dark Arts since they wrote a paper in DADA about lycanthropy). Not to mention his time in the first Order of the Phoenix as well probably meant he had some good experience under his belt that he could share.

Now for the LEAST deserving, this is hard.. merely because so many of them were terrible... but I think my answer would have to be Lockhart. The dude had ZERO clue what he was actually doing. And all of the things he claimed to have done, he hadn't ever actually done so the question would then be did he actually have any experience in DADA? Was he even DECENT at it? My assumption would be no given the mess ups he showed whenever he did attempt to show magic. He was definitely not fit to be a Professor.