I'm torn between the Dark Phoenix storyline and the Sauron/Savage Land/Magneto storyline from the end of the original sixties run. I think on reflection, though, I'd probably have to go with Thomas and Adams over Claremont and Byrne.

I've always felt that the X-Men were strongest in their aftermaths. The Dark Phoenix Saga was impressive, but how the team coped after the fact, fully feeling the weight of the loss like real grieving humans -- that defined the team for me. Or the aftermath to the X-Cutioner's Song, where Jubilee takes Professor X rollerblading, or whichever event it was that had Kitty reading to a dying Ilyana in the aftermath. Powerful stuff.

I am very sorry that they made a travesty of it for the movie... 

I'd have to say "Days of Future Past". It might be an obvious choice now, but I still remember the impact it had on me when it was first published. It was probably the first "dark" superhero story I ever read.

I'm also quite fond of the Alpha Flight/Wendigo story that preceded it. A great showcase for the friendship between my two favorite X-Men at the time.

It's also the story in which the X-Men learned what Logan's name was, following a casual remark by Heather Hudson. It's that kind of small detail, that progressive building up of the X-Men universe, that helped make it such a great place to visit in the 70s and early 80s.

For pure story the Thomas/Adams Sentinel storyline. I also tend to enjoy certain issues more than storylines. Issue 104, when they run up against a restored Magneto, is a great one from early in the Claremont run. At the time, #96 was a good one, in the aftermath of the death of Thunderbird. We meet Moira MacTaggert, see Wolverine in trouble and get a set up for the Shi'ar storyline. Said Shi'ar stuff was quite good, while Cockrum was onboard, crafting his own Star Wars. The abrupt switch to Byrne kind of undermines the ending, for me. 109, with the fight with Guardian in the park, is another. That's when I first noticed Byrne depicting Wolverine holding his mask like a helmet. 126-128, with Proteus, is outstanding. It has some of the mindf#$% caused by Mastermind (as Jason Wingarde), plus Wolverine gets the stuffing ripped out of him and is closed to snapping. Some great Moira stuff, pretty high stakes, and some nasty business for Colossus. Kitt's inro in 129 is quite good, though the Dazzler is underwhelming in the next. I thought the character had potential, way back, but not with the disco schtick. 138 is an emotional journey through the history of the team and very powerful. 151-152, with the return of the Hellfire Club, was a neat little two-parter. 153 is one of my all-time favorites, for Kitty's bedtime story. It's both a great adventure, funny, and touching. Plus, Cockrum gets to go to town on the art.

I read up through Paul Smith's tenure on art; but, the 150s were about the pinnacle for me, too. After that, I felt Claremont was really starting to repeat himself.