The best ever iPod (for sound quality) was the 5th generation ‘Enhanced’ version. But it has the same Apple model number as the 5th generation (A1136) . Here’s how to tell a 5th Gen ‘Enhanced’ iPod apart from a standard 5th generation iPod.
Here are some frequency response graphs for the various models of iPhone.
For me the number one criteria for buying an iPod is audio quality. Some people will go for looks, size, or battery life, but for me it’s which iPod sounds the best – not just through the headphones but also into my stereo.
This article discusses the reasons for the difference in quality of the sound output of the various models of iPods and iPhones, which iPod sounds the best, and why. This article mentions old iPods but it is not out of date. I personally think that the 10-year-old iPods mentioned in this article are better than the current 2016 lineup in terms of sound quality.