The first issue of Uncanny X-Men I ever picked up was #174 and I was instantly hooked. I can't explain what it was that chimed with me but I wanted to know as much about the team as I could find out, so picked up quite a few back issues / reprints.
Even though the Mutant Massacre resulted in a big change to the status quo, I still remained a fan because the new team members and writing kept me interested.
For me, the rot set in after the team went through the Siege Perilous and everything I had enjoyed ceased to exist. I stayed with the title, mainly buying out of habit, until Bishop arrived but that was the point I finally gave up. (The introduction of the second title and the plethora of other mutants titles was also a factor).
Since then, I have dipped in to the title sporadically but the only run I have really enjoyed since is X-Men #187 - 205 (collected in the Marauders / Blinded by the Light trades).
The current Krakoa stuff does absolutely nothing for me.
So, my favourite X-Men era is Uncanny #167 - 251 and New Mutants #18-62.
How about you?