A lot depends on your preferences, of course. I want a compact phone (130 grams or less, 4.7" or less) running Android that is water resistant, has an excellent camera and that has an extensive set of LTE bands for international travel. These requirements eliminate almost all smartphones. For example, although the Moto G is a good value, the camera is very mediocre, it's not water resistant, heavier than I would like, and the set of LTE bands does not meet my requirements.
I am about to pull the trigger on the Sony Xperia Z3 compact (129 grams, 4.6"). It has outstanding reviews, a great camera, expandable memory, and retails for about $500. It also has a fast processor and large amount of RAM, unlike a lot of the smaller size competition. It is just becoming widely available in the USA this month.
IMO, the Nexus 6 is really a tablet disguised as a phone (184 grams or 6.5 oz., 6"). I just don't want to carry around a brick in my pocket, but for someone carrying a handbag all the time or something I can see how it would work.