1. project86
    "Best smartphone for audio?"
    Pros - Excellent sound straight from the headphone jack, slick OS, digital output
    Cons - Just the usual issues with paying full price for an unlocked phone rather than a carrier subsidized mainstream phone.
    For those interested in using their phone as their main audio source, the Meizu MX 4-core is my top recommendation. It ticks all the boxes for a quality phone - responsive interface, nice screen, camera that is actually useful, good amount of memory, etc. But there are plenty of nice phones out there. What sets this one apart is the audio quality - it's really good!
    Straight from the headphone jack, this thing sounds superb. It's clean, clear, and has a black background. It has a good amount of volume too, able to comfortably drive plenty of full sized headphones from Ultrasone, Grado, Audio Technica, etc. The low output impedance (~3 ohms) means it works well with most IEMs too.
    Also worth noting is the SPDIF output capabilities. Using an adapter (that Meizu is still finalizing) the MX 4-core can be used as a transport connecting to an external DAC. I successfully played standard and hi-res material, and the MX feeds it bit-perfect to the DAC. Very useful. 
    See HERE for my complete in-depth review. Bottom line - this is a very competent phone, and also an excellent portable audio device.