I watched through the X-Men movies earlier this year for the first time in what felt like forever, but it wasn't until I had made it through them all(at least in the best order understood to watch them in at the time) that I realized the truely most enjoyable order for them to be experienced. Now bear with me because you X-Men Origins: Wolverine fans might get a little upset. The best and least confusing method to enjoy these amazing films goes, I'm my opinion, as follows:

X-Men

X-2

X-3: The Last Stand

The Wolverine

X-Men: First Class

X-Men: Days of Future Past

Logan

X-Men: Apocalypse

X-Men: Dark Phoenix

Deadpool

Deadpool 2

Legion (series)

You'll notice I did not include X-Men Origins: Wolverine at all, and that's for good reason. Hear me out. The film is jam packed to the brim with continuity issues. It has so many from so many different angles that you can hardly keep up trying to explain how it fits with the rest of the timeline, because it doesn't. From Sabertooth and Logan not recognizing or addressing each other as brothers during their confrontation(s) in X-1 to the appearances of other characters that would later arise in the series who's ages or story don't match up with their former appearance in Origins, there's a lot of issues to notice. Obviously the film could have been written better, as not to conflict with everything else they had planned for the timeline, but it wasn't, and when you take it out of the timeline and simply accept the fact that it was a poorly written film that has no structural value to the core story arcs that follow it chronologically, it gets much simpler and easier to enjoy the rest of the films without confusion. Which honestly still leaves you with eleven great X-Men films to watch back to back without confusion. If you insist on watching Origins I suggest you watch it before X-1, but I highly recommend you delete it's existence from your brain(lol).

As for any other questions about this timeline order. I put the original trilogy first because that's what we were intended to watch first, and keep in mind this is coming from a perspective of relevancy. As I am someone who watches Star Wars in chronological order(from episodes 1-3, Solo & Rogue One, 4-6, and so on). The events of First Class(a story that revolves around how the X-Men were formed through the personal backgrounds of young Prof. X, Megneto, Mystique, and friends) are not relevant until after the events of The Wolverine. It is not until after Logan's journey in Japan that learning of the past becomes necessary in order to lead up to Days of Future Past. Our introduction to these younger versions of the founding X-Men is meant to be a stepping off point to the much grander arc that takes place in DoFP. It is best to have watched First Class directly before watching Days of Future Past. Wolverine has great success in the time traveling mission he's sent on, reverting the mutant-murdering-sentintel-filled dystopian world from what it had become as we saw it at the beginning of DoFP to a much more pleasant reality where, fortunately because of the changes to events in time, the events of the original trilogy as well as possibly the events of The Wolverine, all happened slightly-to-very differently, leaving Scott and Jean alive, and the X-Men given a happily ever after at the school for gifted youngsters, teaching together as professors again. This catastrophic change in events in time not only adjusted the reality time traveling Logan was from to a more enjoyable one, but it also left the past he jumped back from to branch into it's own new timeline, which we'll get back to.

But this aforementioned happily ever after for the reunited Logan, Jean, and Scott, and the other X-Men does not last forever, as many years pass and eventually Xavier becomes elderly and unstable. Prof. X has a psychic old-guy episode and accidentally kills nearly all of the X-Men, Logan being one of the only people to survive, because of his abilities. This carries on into the plot of the film Logan which absolutely is the end of what you can call timeline 1 or A.

Now we return to the new timeline that carries on in the reality left behind by time traveling Logan, which jumps right into X-Men: Apocalypse which is directly followed by X-Men: Dark Phoenix of course, transitioning to an end to 20th Century Fox's X-Men timelines with Deadpool 1 & 2. There is even a scene in Deadpool (2 I believe) that confirms both Deadpool films take place in this same reality which you can call timeline 2 or B.

And that's all folks. I spent an immeasurable amount of time studying the films plots as well as scouring all corners of the internet to read other people's explainations of how to watch the X-Men movies, and after coming to the realization there is still no formed consensus I thought I might throw my penny in the wishing well too. Thanks for reading! Hope you enjoyed!

Also, please comment and share as I'd love to discuss this with any X-Men movie wizards that are higher level than myself. If anyone would like to try out this watch order and get back to me I'd love to hear what you thought.

Edit: The series Legion goes right at the end of the movie timeline. You won't regret checking it out.