First let's look at Evil Dead, the quintessential B horror film. The movie starts out creepy enough, a good location of an abandoned cabin the woods with dripping pipes and an evil history of the cabin. There's some decent camera work and all that to create a fairly good atmosphere, then shit gets really crazy.
First we have the tree rape scene. I will say nothing more, because anything else is extraneous, it is truly horrifying. Then when the girl is recovered from this traumatic experience she becomes a weird zombie thing and is trapped in the cellar, though not before biting one of the main cast. One of the things I really liked about the film is that Bruce Campbell is not the obvious protagonist, at least not a first, he seems more like the guy who dies in self sacrifice last or something.
The others die or get zombified relatively quickly after the event, and then it just becomes a matter of how much liquid they can spray on our hero, as well as just how much violence they can fit in, while still maintaining the eensiest bit of horror and tension. It's pretty much all gone though, especially once they get to the things just attacking from every direction, including a zombie stabbing a character with a pencil, it's just kind of silly.
That's the movie's strength though, and especially where it differs from Nightmare. It embraces the silliness, and the gore and all that and becomes a much better movie and series for it, unlike some of the later Nightmare sequels, which turn the former terrifying antagonist into a wisecracking joke of a villain.
Talking about Nightmare it is a genuinely good horror film which features Johnny Depp (In a first role) being completely obliterated in a geyser of blood. The way the film builds tension is like all the great ones, by not showing the villain, always cloaked in shadows and unknown until the climax of the film he was all the scarier for it.
I mean sure, nowadays everyone knows Freddy Kreuger, something the remake took for granted and was much the worse for it, but then it was such an effective scare with someone targeting us when we are at our weakest, asleep. Using nightmares as a tool added an extra depth to the scare, because by having nightmares about the film, an intended consequence, it added more impact to them because it was possible you would die in real life. Not actually, but you know.
Anyway I don't have an enormous amount to say about either of these films, other than to recommend them to all lovers of horror and thriller too, and especially the connoisseurs which prefer something a little dated, and with just that tad bit of humor, though not too exaggerated like Nightmare 4-8.