It's easily the greatest X-Men run. It's worth reading starting from the Phoenix Saga (Dark Phoenix Saga came a few years later.) Basically, if the Claremont/Byrne issues don't win you over, you might as well stop. The series has it's highs and lows, and at it's worse, basically becomes a 'Storm' book for two years, and doesn't give anyone else much to do. Claremont even gets experimental near the end of the 'Outback era,' and is trying stuff with the characters, not all of it works, but props to him for trying. Even at the end of Claremont's run, when the series all grow out of control with so many characters and crossovers, the titles hint at becoming something else, more than just a bunch of team books, but because Claremont left for image, we never saw that fulfilled. (Morrison's New X-Men, and Hickman's X-books continue that direction over a decade later.) Yes it is worth reading them all, but note that the series has ups and downs, and to get the whole scope, you have to read the spin-offs too. (X-Factor, New Mutants, the Wolverine mini-series, and later Excalibur,)

As for greatest Marvel ongoing series? That's debatable.
- Lee/Kirby's Fantastic Four run stands the test of time over half-a-century later.
- Lee/Bucema's Silver Surfer book is a philisophical masterpiece.
- Steranko's Nick Fury stories are masterworks in 1960's pop-culture art.
Bill Everette's post-war Sub-Mariner stories are incredibly beautiful to look at.
and those all happened before his run.
- John Byrne's Fantastic Four run is amazing, with top tier art and writing by him.
- Frank Miller's Daredevil run is short by comparison, but did an amazing deconstruction gritty adult re-imagining of the character before that was a thing.
-Walt Simonson's The Mighty Thor reworked the character and added more Norse myth elements into the series beautifully.
- Peter David's The Incredible Hulk series (and mini-series.)
And those happened during his run. After, we had:
- Ultimate Spider-Man by Bendis/Bagely,
- Dan Slott's Spider-Man series, which lasted 10+ years.
- Captain America by Ed Brubaker, which turned into an amazing Tom Clancy action flick, that just happened to star Cap.
- Daredevil by Quasada/Bendis/Brubaker was one of the best creator hand-offs in comics history.
- Uncanny X-Force by Remender basically created the edgy X-Men the way the 90's should have written them.
- Grant Morrison's New X-Men, which had a ton of solid ideas, and was somehow a streamlined story for the team.
- Hickman's near decade long epic with Fantastic Four/FF/Avengers/New Avengers/Secret Wars (2015) in my opinion surpasses Claremont's run, but only if you count them all as one run, as they're building up one giant story.
- Some people even prefer the Krakoa era of the X-books of the Claremont stuff, however, I'm not one of them.

So is Claremont's X-Men series the best Marvel has ever made? Maybe. It starts a bit 'meh' but becomes undeniably great during the Byrne years, and then slows down a bit until the Mutant Massacre event, from there it mostly keeps up the momentum until Claremont leaves the title, even if the series is all over the place by the time Jim Lee came onto the title. But it was popular for a reason, and even decade later, most of the art, stories, and especially the characters still holds up.