New X-Men Volumes 1-3 (Hardcover)- Grant Morrison's epic run on the X-Men, Controversial, but accessible, and I loved it.Originally Posted by CLavery
Mark Millar's run on Wolverine- I just love this book. Wolverine's the dangerous badass he was always supposed to be.
X-Force isn't really a core X book, but it's enjoyable as all hell. X-Statix has great moments, but simply isn't as good.
The best current book to pick up is Astonishing X-Men, as it's accessible, has a great creative team, and it's pretty much the flagship title right now,
If you want the X-Men stories ever, I'd recommend the Essential X-Men Volumes 1, and 2, which are packed with the defining (and best ever) X-Men stories. I'm honestly convinced the book was never this good again until Morrison.
The Best of Wolverine hardcover includes defining stories of the defining X-Men.