Having stock Android is awesome. After months of use, still just as fast as the day I got it and battery life is still outstanding. Can't say that about any other Android phone I've owned.
Specifically for the 6P, I'd say the fingerprint sensor on the back. Thought it was awkward when I first read about it but now it's natural and super convenient. I even did a "Jedi mind trick" to unlock it when I showed a friend (who uses an iPhone). He was quite impressed, lol.