For transport only it depends on how you listen to music:
Streaming apps - I would just use your smartphone with the appropriate app to ensure bit perfect pass through.
Local music files - I would go with a Sony A series Walkman. They are fairly inexpensive and battery lasts a very long time. Also if you have a smartphone with enough storage or supports micro SD Card, then I would just stick with the smartphone.
It also depends on what you connect to your transport. If it's a portable device then the above recommendations would work. If you need a transport for a desktop class DAC, then I would also look at an iPad or Android tablet since the UI experience is much better than a smaller screen. The only thing that can be a major pain for Android is finding a tablet where the sound processing can be bypassed or else you are stuck with 48 kHz sampling rate.
As for your last question, once you get to M17 class of DAP it is far more a matter of your specific gear and personal taste. There is no universal "best" DAP. Even when you factor in SQ, then you have to factor in how you will use a DAP for your own personal cases. I use to think I needed a TOTL DAP when I fly on a plane or a long drive when I'm a passenger. It turns out I would rather catch up on TV or movies instead since it makes the time go by faster. I am much happier in that scenario with my wireless Sony IEMs with ANC. My personal use case for DAP is at home, but moving around to different rooms since I don't have a dedicated music room. In that regard I prefer the M17 since it sounds great on all my headphones and I don't need to be close to a power outlet. I doubt I will ever travel with my M17.