A viewing order for the X-Men franchise.

Film Bryan Singer and co-screenwriter Simon Kinberg have taken some time out during the promotion for X-Men: Apocalypse to explain how time travel has fixed the X-Men universe. [link via]

I thought it was pretty well explained on the Days of Future Past Extended blu-ray but it essentially boils down to a similar approach to JJ Abrams in Star Trek except with Wolverine as the emissary between the timelines rather than Spock.

Spoiler warning for Days of Future Past.

The events of Days of Future Past have wiped out:

X-Men
X-Men 2
X-Men 3 (or whatever it's called in your end of the world)
X-Men Origins: Wolverine
The Wolverine

And now the official X-Men Cinematic Universe contains, in chronological order:

X-Men: First Class
X-Men: Days of Future Past
X-Men: Apocalypse
Fan4stic
Deadpool

BUT

Wolverine consciousness has passed from one timeline to the other.  So the Wolverine walking around in the future scenes in Days of Future Past is the same one who turned up at the beginning of X-Men.

Which is were it gets complicated because for him to take control of the alt.Wolverine in that future means the mind of alt.Wolverine has to be wiped over; he shows no signs of actually remembering anything that happened to this other Wolverine up until that point.

Luckily he's not apparently in Apocalypse so we won't actually see the alt.Wolverine walking around, having a life before the "real" one comes along and murders him later.

BUT it does mean that it's possible to have a chronological viewing order for the X-Men films if you want to see them from Wolverine's perspective (give or take some suspension of disbelief in regards to Sabretooth) (you could just assume them to be two different characters with the same name which does already happen a lot in comics):

X-Men Origins: Wolverine
X-Men
X-Men 2
X-Men 3 (or whatever it's called in your end of the world)
The Wolverine
X-Men: Days of Future Past

My preference is for the Extended Cut of DoFP because it nicely bookends Wolverine's friendship with Rogue.

This leaves out:

X-Men: First Class
X-Men: Apocalypse
Fan4stic
Deadpool

unless you want to sit through First Class just for his cameo.

Presumably the next Wolverine film will be set after DoFP.  As Singer says in the interview, what he's now making are prequels to the alt.future scenes in DoFP.

Of course as a result of all this, Hugh Jackman once took a break from filming DoFP to promote The Wolverine which was essentially being wiped out in franchise terms by the very film he was making.

Except if you look at the series instead from Wolverine's perspective, the closing scenes of DoFP have an even greater emotional resonance because of one of the subplots in The Wolverine.

Perfectly simple really.