It's definitely hard to choose between the three series, since I like all three (though with Halloween I haven't seen parts 3, 8, or the two Rob Zombie versions).
I think I'd have to say Friday the 13th though. they did the best in evoking real atmosphere on a small budget, I think part 4 is my favorite one (in case you don't remember, that's the one with Corey Feldman). F13 is also the best one at exploiting the "dead teenager" formula in that the characters are only introduced to either die, or kill Jason. If I can set some money aside in the near future I may splurge and get the whole blu-ray series set.
Of the Friday series, I quite like the first one. I think it's actually pretty scary. It's easily a riff on Halloween (as even admitted by the writer and producer), but it's still a fun little movie. Jason Lives is also a good one. I'm actually pretty convinced the idea for Scream came from this movie, as it contained a lot of references to horror movie cliches.
Plus, this is the one that had that awesomely absurd triple be-heading.