What's a nice smartphone for a security/privacy-conscious person?

Privacy-conscious Bill is looking for a new smartphone. Doesn't have to be super-fancy or expensive (e.g. Blackphone, Motorola AME 2000) or hipster (e.g. Jolla, Geeksphone, Openmoko) or anything. A nice, little, widely available consumer phone will do, as long as it's friendly to custom ROMs like Cyanogenmod.

A Nexus phone might be a good option. Easy to customize, a dedicated community supporting it, reasonably priced, and nice. The 5X is decent, and last year's 6 is on fire sale everywhere, although I wouldn't call it "little."