This is similar to the thread about what ten things you hate the most about the X-Men lately (or however that was phrased). But much more restricted as to options.
You can go back to any publishing year between 1963 and 2017. (1963 is the year the X-Men first appeared.) Whatever year you pick, the stories that were published up to December of that year are still in continuity - but all of the stories that were published subsequent to December of that year never happened. So if you pick 1963, issues 1-4 of the original X-Men title will remain as still having happened - but everything after issue 4 goes up in a puff of smoke and will never be referenced again in any comic book forever.
What year do you pick? 2000? If you pick 2000, then the Morrison era (which began in 2001) never happened, nor anything following it.
Or maybe you want to pick some year in the 90s. You could pick 1989 and wipe out all of the 90s and all of this century - or maybe 1995 and wipe out Onslaught and everything after it.
I've been an on-again off-again reader of the X-Universe so I don't personally have a really good answer, although I certainly, without hesitation, would endorse the choice of year 2000, to wipe out every X-story published in this century. I'm sure some good stuff would be lost, but oh my God the amount of dreck that would be vaporized! And I say that, despite having actually enjoyed the recent X-Men/Inhumans cross-over - which probably I only enjoyed because I was entirely oblivious to the umpteen continuity errors. I also was pretty OK with the Blue and Gold books for a while, though I finally got bored with them both, I think mainly because Secret Empire interrupted things and all momentum (for me) was lost. I would sacrifice the good to get rid of the accursed dreck!
How about you?
Because I would never want to give up Peter David's second X-Factor run, so in that case, I would say the second that last issue's lat page hit the printer.
I think it is the closest we will ever get to Claremon't golden age, again.
If it is JUST the X-Men, I would say, some time before the whole mansion became a school with a large number of students (Morrison, I suppose).
That never worked for me, and to this day I want any X-Man to say,
"Look, I didn't sign up to teach Algebra to teenagers, I signed up to be a superhero... and maybe some pro-mutant P.R.. For that sake, I don't think any of us have teaching licenses or degrees. And our mansion gets attacked or destroyed by Sentinels, super-villains, or intergalactic warriors every other week, WHY DO WE HAVE KIDS HERE!? We're going to go to prison."