What smartphone has the best sound quality?

Apple iPhone 4 Black Smartphone 32GB (AT&T)

Product Description 3.5" TFT capacitive display, Scratch-resistant surface, Multi-touch input method, Accelerometer, Proximity, Three-axis sensor, 32 GB, Wi-Fi 802.11 b/g/n, Two Cameras, GPS, Multitasking capable, A4 CPU Product Features * Do... read more...

I would say iPhone 4 or 4s would be your best bet for music quality. I mean you get your iTunes library, and iTunes is the world’s largest digital entertainment store. The sound quality is excellent. It has a very wide frequency response of 20Hz to 20,000Hz. It also supports a wide range of Audio formats, including AAC (8 to 320 Kbps), protected AAC (from iTunes Store), HE-AAC, MP3 (8 to 320 Kbps), MP3 VBR, Audible (formats 2, 3, 4, Audible Enhanced Audio, AAX, and AAX+), Apple Lossless, AIFF, and WAV. You will be very happy with this phone.

✔ Highest voted answer

Reply · Send thanks · posted 99 days ago · 3 people agree

HTC A6262 SmartPhone Unlocked--International Version with No Warranty (White)

HTC Hero is our first phone to embody HTC Sense - an intuitive, seamless experience built upon three fundamental principles - make it mine, stay close, and discover the unexpected. With HTC Hero, personalization reaches a level never before possib... read more...

Reply · Send thanks · posted 93 days ago

HTC Sensation XE and XL come with special equalizer and earplugs by Beats-Audio by Dr.DRE. If you are not a hardcore apple-user I would not go for an iphone and be doomed to use itunes.
Concerning the supported audio-formates you can't go wrong with android either. Most phones and music-apps support FLAC (Free lossless audio codec) which is among the most popular HQ-audio codecs!