A few of the Friday the 13th sequels (Parts 4 and 6, Freddy vs. Jason, and even Jason X) are generally regarded as the best in that series. Admittedly, that's a pretty low bar.
Nightmare on Elm Street Part 3 is usually considered the best of that series as well.
The original Halloween, on the other hand, set such a high bar not just for the franchise but for horror movies in general that any sequel would have a hard time even approaching it much less besting it. Halloween 2, H20 and the latest one were pretty good. Part 4 wasn't terrible. None are as good as the original.
Part 6, though ... my god, did that thing suck ass. The plot is that Michael Myers has raped and impregnated his niece, in order to conceive a baby that he can then kill as a sacrifice to a druid cult that has controlled him since birth. Most of this is to pay off plot threads started in Part 5. And it only gets worse from there.
Someone mentioned that Halloween 3 was supposed to take the series in a different direction, but it failed and they had to bring back Michael Myers. The funny thing is, Halloween 3 sort of did what it was supposed to do. The series has become an anthology, it just happens to be an anthology of stories starring Michael Myers as the villain.
There are, IIRC, six different timelines now in the Halloween franchise, which is at 11 movies and counting. The first two movies are canon, and then the others all branch off from there.