The first part (10 out of 10) is a true masterpiece of the horror genre. The second (7 out of 10) is a worthy sequel. The third (3 out of 10) is better forgotten. The fourth is only as good as the blood, Pleasence and the ending. The fifth (5 out of 10) is a standard sequel, copying the previous parts.
The main good thing about the first movie was that Myers was a maniac who kills for no reason. He wasn't conceived by maniacs, he wasn't drowned in a well, he had a pretty decent childhood. But the makers ruined everything by making this movie.
This movie is pure thrash that 'Halloween' didn't deserve. The script was only worse in part three. The actors are terrible, except for Pleasence (too bad, but that was one of his last roles). Plus, the movie contains a few bloopers (Loomis' burns are too badly healed).
The film does have some pluses. A spectacular appearance by Myers in the beginning, the makeup, the soundtrack, the nudity, a couple of cool kills. But it's not enough to make up for the disappointment of what was made of Michael. Now he doesn't seem to be the embodiment of evil, but just another maniac created.