For me, it's the Hifiman Shangri-La Jr. I can listen, and have listened, to that headphone for hours at a time without ever wanting to stop. It has well-extended treble that's very airy yet smooth, and for me, airy treble is relaxing. Soundstage is quite good and sound placement seems natural to me (in comparison, I thought the HD800S was disproportionately wider than it was deep in soundstage), but above all, the SGL Jr is very open sounding, creating a convincing illusion that sounds are radiating into a space around me rather than being shot directly at me. I love an airy, open, slightly sparkly sound in headphones, and the Jr delivers that in spades. It's not the most "natural" tonality, but it works well for classical music, while for vocal EDM the midrange tuning pulls vocals back (overly forward vocals quickly annoy me) and gives them a bit of a "breathy" or "whisper-like" character which I enjoy. I run it on the Ferrum Erco DAC to give it more dynamic kick and a modded Stax SRM-006tS that adds a slight bit of warmth to the sound.
Fatigue-free? Yes.
Laid back? Sort of, at least it is in the ways that matter to me.
Natural? Not quite, but natural enough.
Spacious? Definitely!