This is actually going to be a somewhat simplistic post but I actually think a simple run-down is the best way to illustrate my opinion.
1. First appearance of Alpha Flight. This is great for two reasons. First, it's Alpha Flight. Second, it pays off the previous appearance of Guardian and reinforces Wolverine's outlaw status from the Canadian government.
2. First appearance of Kitty Pryde.
3. First official appearance of Hellfire Club, including White Queen/Emma Frost, a VERY important character in the X-men. Again, this is extra-awesome because the existence of this group had been hinted at YEARS earlier and paid off in a huge way.
4. Dark Phoenix saga. Again, awesome both on its own, but even better because it pays off something hinted at for years in the book.
5. Days of Future Past, a two-part story that sees the introduction of the New Brotherhood of Evil Mutants led by Mystique (created previously by Claremont in Ms. Marvel). Also introduces Rachel Summers (the new Phoenix).
6. First appearance of the Brood.
On average, that's a major comics event worthy of a 12-issue Brian Michael Bendis mini-series every 4 issues.