ZTE spent the last few years building low-cost devices for carriers, but 2015 saw the release of the Axon Pro. This phone had flagship specs and a pretty nice build of almost-stock Android. ZTE promised it would keep this phone up to date, and it's following through with a Marshmallow OTA. It took a little longer than expected, but you can grab the update now.
Android 6.0 on the Axon Pro comes with all the standard Marshmallow goodies including Doze, Now on Tap, and more. ZTE also calls out a new app drawer, which I assume is a reference to its slightly tweaked custom launcher. The addition of fast scrolling and searching sounds like Google's launcher, but the Google Now Launcher is not tied to the 6.0 update, of course.
The update is 835MB in size, so finding some WiFi is a good idea. You can trigger the update by going into the system update menu, but only on the Axon Pro. The more budget-friendly Axon is not being updated at this time. The timing of the Axon Pro update is nice considering this phone is currently $70 off via Amazon, though it's selling out fast.