Every Defence Against the Dark Arts Teacher, Ranked
The best
Remus Lupin is the consensus pick, and it's not close. He's the only teacher who actually teaches the students practical defensive magic, introduces them to creatures they'll face in the real world, and treats every student, including Neville, with respect.
His boggart lesson is the best class scene in the entire series, because it shows a teacher who understands that confidence matters as much as technique. The fact that he's forced out because of prejudice against werewolves makes him the series' strongest example of institutional failure.
The fake Moody, who is actually Barty Crouch Jr using Polyjuice Potion, is the most complicated entry on the list. He's a Death Eater, and yet he teaches the students more useful magic than most of the real professors. His lessons on the Unforgivable Curses are genuinely educational, which raises uncomfortable questions about whether a good teacher with terrible motives is still a good teacher.
The middle
Snape finally gets the job in book six, and he's competent but characteristically unpleasant. He knows more about the Dark Arts than anyone else in the building, which is exactly why Dumbledore kept him away from the subject for fifteen years.
Quirrell is an interesting case. He's presented as incompetent, but he was harbouring Voldemort on the back of his head for an entire school year, which suggests either remarkable acting or remarkable inattention from the rest of the staff.
The worst
Dolores Umbridge is the most hated character in the series, more hated than Voldemort in most fan polls, because she's the kind of villain people actually encounter in real life. She uses institutional power to punish dissent, refuses to teach anything useful, and physically tortures students with a quill that cuts words into their hands.
Gilderoy Lockhart is a fraud, but at least he's an entertaining one. His lessons are useless and occasionally dangerous, but his vanity is so transparent that the students figure him out within weeks. He's the only teacher who's genuinely funny, which is why his memory-loss ending feels like exactly the right punishment.