Unfortunately to properly answer your question "what is the best portable music player to have?" a few simple questions need to be answered first since there are many, many different portable music players available that serve many different needs.
So first how will the player be used, i.e. in what type of situation and environment? Will it used while commuting to and from work and what type of commute? Bus, commuter train or noise urban rapid transit?
Do you want a one box player, i.e. no external amp or DAC?
What type of headphones - over the ear, on the ear or in ear monitors? However you have answered this question - the DUNU DN-2k, which are in ear monitors and which, from what I can gather, are not to difficult to drive meaning that an external is not necessary although you may want to use one anyway.
What type of files will you be using? For example: Mp3s, m4a (Apple's lossy format), flac, Apple lossless
Answering these simple questions will greatly help to narrow the available choices so that the decision becomes much more manageable.
So for example if one wants a one box player that will used with in ear monitors in a relatively noise environment (e.g. while on the NYC subway) and listening to mp3 files than pretty much any late model smart phone will do the job quite nicely.
Or if one will be listening to high resolution (24bit/48kHz and above) flac files in a relatively quiet environment (commuter train or bus) using full size over the ear headphones then a dedicated music player would be suited or a player which offers digital output combined with a portable DAC/amp.
I hope this helps to get you at least pointed in the right direction.
