I love this film to death. I think it is among the best SWs ever -- so, yes, A-Class. Obviously, the Leone films come first and then Great Silence, but I think this is my favorite after those 4 films.
(Don't read further if you're worried about being spoiled...but I'll be somewhat vague just in case you want to read)
What I love about DAY OF ANGER is unusual hero character in Scott Mary. This way he's treated and abused then rises up is just great. Very against the grain, and cool. This isn't the usual silent stranger tough guy; Scotty Mary has to develop his grit. He's learning to be "bad" so to speak.
Also, I love Van Cleef's performance in this film. He's just really dark. I read a review once before, that I think is bang-on, which compares the student/teacher relationship to Star Wars. Van Cleef is trying to tempt Gemma's character into being just like him -- cruel, mean, etc. And the best part, to me, is that I couldn't really blame Scott Mary if he did side with Van Cleef after the way he's been treated by the townspeople.
The direction, score is great. I love the dark humor in the film as well. It is a very unique film in genre. It may be a little "simpler" than other spaghettis, but I do love it. I love Big Gundown, too, although I prefer this one to that one in terms of Van Cleef films. I enjoy Death Rides A Horse, but I think DoA is without a doubt superior.
I'm sure I haven't seen as many SWs as most here, but this is definitely in my top five for now.