The first one is just a solid horror movie, the second one is cool because it delves into the mythology more, in the third one they just go buck wild and basically throw everything out the window but it's still kinda fun in an over the top kind of way, the fourth one they went back to the mythology but they also went to LOL SPACE so yeah there's that (still scared the piss out of me as a teenager

). After that the series went DTV, the fifth one I didn't really like at the time but in retrospect it seems better after seeing all the other DTV sequels, the 6th one I got sucked into thinking it might be good because it had the chick from the first two movies (nope, it was bad). Anything after 6 shouldn't even be brought up in polite company.
Basically I would say 1 & 2 are the only ones that are "good", after that proceed at your own risk.