Uncanny 140 for me. I guess there's something about the age of 9-10.... as that's how old I was when I got this book in a trade with my neighbor. It's a stand-alone story, which I think played a big role as it was already a couple of years old, so when I went hunting for more I believe the run was up to 168 or something - but I've managed to fill in the blanks over the years. ; )
AWESOME Byrne cover with Wendigo slamming Wolvie into a tree, and the story inside does a wonderful job of spreading the spotlight around to most of the team.
Honourable mentions to Dark Knight Returns #1 and the X-Men graphic novel.