The first HTC smartphone to run Android 6.0 Marshmallow is the One A9, as you may know. But the first HTC handset to be updated to the latest version of Android is in fact the One (M8) Google Play Edition (GPE), the Sense-less version of the Taiwanese company's flagship for 2014.
Since this is a GPE model, updates are handled by Google and only sent out by the OEM, HTC in this case. That explains why the update's been so quick to roll out. The download for the One (M8) GPE is around 450MB in size.
Next up we should see Android 6.0 on its way to the One M9 and the Sense-filled One (M8), with rollouts reportedly starting before the end of the year. And then, in 2016, Marshmallow will also make it to ten more HTC devices.