If I could only have one, the it would involve a lot of compromise. It wouldn’t actually be my favourite or the best at anything in particular.
My preference is Speakers > Headphones > IEMs
However, you said no speakers and as much as I prefer headphones, it is just too hot here for headphones in summer. I also travel a lot and with all the stuff I end up needing to take with me, headphones take up too much space.
So that leaves me with IEMs…
I have sets of IEMs that I prefer for certain things, those that I feel are “best” at something (out of the ones I have tried of course), but I really don’t think I would pick any of them as a “only” set. I listen to so many variations of music that I would aim more for a “jack of all trades, master of none”.
So I need:
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Something comfortable for many hours
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Something that I can wear in public without worrying about them looking expensive (even if they are not) because I travel to a lot of port cities which are many times not the nicest of areas
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Something that doesn’t irritate me with badly recorded music but also does justice to well recorded music
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Something that I can throw in a bag and not worry about losing or damaging it, basically not too expensive and not irreplaceable
So, considering all those factors (plus the fact that I am not willing to just have some TWS), from what I own (and have heard) I would probably pick the Sennheiser IE600.
As far as source, I would need it to meet the same criteria as the IEMs, along with not being too large. I discard DAPs because they take up space that is already at a premium. I also discard the Gryphon, as much as I love it, for the same reason.
I would pick a dongle and out of the ones I have tried, it would either be the Questyle M15 or the Aune Yuki. I like both but with the IE600, I would probably go with the M15.
That leaves just the cable, which I am not too fussy about, as long as it is decent. I would like a cable that is not too thick, is not rubbery and is not the same cost as both devices chosen above. The problem is finding one that fits with the IE600 (due to those stupid mmcx connectors). Seeing that I am saving so much by cutting back on gear, I would probably go for something custom made (and break my cost rule), if possible by Viking Weave 'cos they are great guys and make great cables.
That was a lot of thinking out loud (or typing while thinking) and it has brought me to a conclusion that I wouldn’t have guessed.
Phone/Tablet/PC > Questyle M15 > Sennheiser IE600
However, I reserve the right to change all of that to a Cayin N7 + Storm (if I ever get to hear Storm and win the lottery 
)