Chris Claremont hands down. And it's pretty obvious why. It starts with the fact it was a 17 year run, but everything you know about the X-Men came from him. And newer writers are STILL recycling his stories. I don't consider anything he did after leaving the book in 1991 a part of this legendary run though, because he's now completely insane.
I'll give Peter David props for his X-Factor runs too though.
The absolute worst was Grant Morrison.