How do the three compare, in your opinion?
It depends on your needs. For me, the Nexus 6P and the HTC 10 are the best smartphones. I really liked my Nexus 6P a lot. The software experience, the speakers, the very frequent updates, running Nougat..
The iPhone is for people that want something that works very well out of the box, and do not want to start toggling things, or customizing. It is really a great phone, but for me it is just boring.
The HTC 10 offers the best experience for my needs.
I love using Blinkfeed to get all the latest news. The interface is very smooth, just like the UI on the Nexus. The speakers offer a much better quality compared to the Nexus and the display is really good, despite it not being AMOLED. Battery life is better on the iPhone 6S Plus, followed by the HTC and last comes the Nexus.
With the iPhone I could easily get 6-7 hours SOT. With the HTC I get something between 4-5 SOT and with the Nexus most of the times I got something between 3-4 SOT. Of course your usage might vary.
I cannot really say more on the camera of the HTC 10. I just took a couple of pictures yesterday and they looked great, but the Nexus and the iPhone are also very good.
The biggest difference for me between the Nexus and the HTC 10 is the design and the hardware quality. The HTC 10 is just the most premium phone out there. The Nexus sometimes felt cheap on the hand. It has a nice design, but I prefer the sleek look of the carbon gray HTC 10.
My only caveats with the HTC 10, compared to the Nexus 6P are that the System UI tuner cannot be customized, and that the bigger screen estate of the Nexus enabled better productivity scenarios, especially when editing documents (I like to edit some Excel sheets on the go). The HTC's double tap to wake gesture also does not always work and this is something HTC should work towards fixing.
I prefer the Google dialer, because it has spam call recognition built in and also caller id recognition, by using an Internet database. The Sense UI is great, but grid spacing is just too big. HTC could improve that too by offering an option to change the number of icons that can be placed on a screen.
The notification LED is better on the Nexus. It offers more colors and it is more subtle. The HTC notification led is too strong for me, and it has only two colors.
Overall, the HTC 10 is a great device. Maybe the stand out feature of the HTC 10 is its design, followed by the great audio.