Essential X-men Reading
This is my list of what I consider to be the most essential X-men stories.
That's not to say that these are all of my favorites; there's a few stories on here that I'm not even especially into, while some of my own personal favorites simply didn't make the cut.
But these are, to my mind, the stories that have and most likely will stand the test of time as they are most definitive of what the X-men are about.
Feel free to let me know if you think there's anything I've missed.