nc8000
Headphoneus Supremus
This is actually a difficault question and a lot of the answer depends on the definition of portable. For pure portability I have settled on an iPhone with an iBasso T4 amp and Etymotics HF2 headset the gets swapped between the iPhone headphone out when I need to be able to make and answer calls or the T4 headphone out when I just need music. For better sq but also considerably more bulk the iRiver H-140 optically into an iBasso D10 feeding APS recabled ER4-P is imho very hard to beat and is my travel rig. In my day to day life that is too big to be truly portable.