Halloween is the best of the 3 in my opinion.
Nightmare is a close second but both are part of my all time horror favs. As far as rating the entire Nightmare series I'd say:
Part 1 Excellent
Part 2 Poor, one of the worst in the series
Part 3 Good but I don't like what happens to Nancy, a pet peeve of mind is when survivors of one horror movie get axed in the next one.
Part 4 Good but the same complaint again in regards to Kristen.
Part 5 Ok/Fair This one is a little off to me.
Part 6 Very mediocre/Bad Too much comedy, Freddy has become a clown
Part 7 Ok but not really a part of the nightmare series in my opinion
On the Halloween flicks
Part 1 Excellent/ A classic
Part 2 Ok but the guy playing Michael is just not as good
Part 4 Good I like the story and seeing Loomis back(note I don't count 3 because that's not a real Halloween flick)
Part 5 Good, it kinda falls under the same category as 4
Part 6 Offensive/Unbelievable bad, it's ashame this is Donald Pleasance's last one
H2O Good, I liked H20 and liked the return of Jamie Lee.
Resurrection ok It's a bit of a letdown when compared to the others, I didn't like what happened to Laurie and the whole internet thing was a stupid Blair Witch rip off
Friday the 13th
Part 1
Part 2
Part 3

Jason really starts to take off here.
Part 4

One of the best in the series I think
Part 5 ok the lack of Jason hurts this one
Part 6

The best one in my opinion
Part 7 is ok but the psychic girl thing is kinda stupid
Part 8 is really bad,
Part 9 is a cheap The Hidden rip off
Jason X I kinda like. I like the killer on the spaceship movies.