Well that’s a big queston. There are currently a lot of headphones that I find to be very good in some respects, but compromised in others. So I guess my ideal theoretical headphone is one in very good (or above) class in all respects, and compromised in none. Which is why it not only doesn’t exist “yet”, but probably never will.
It would need to be :-
- open back, over ear
- light enough for long use in listening sessions
- heavy enough to be very durable
- be ideally suited to MY hearing
- be at an “affordable” price.
And that little lot, as we all know, holds some big issues. Not only does something that might sound fantastic to me not necessarily sound fantastic to someone else, especially someone reviewing it, and vice versa. Which makes relying on anybody’s opinion on how it sounds … problematic, and finding that headphone tricky, even if it does already exist.
The same sort of limitation applies to comfort, albeit to a lesser degree. Just because it fits the reviewer comfortaly doesn’t necessarily mean it’ll fit me. After all, we probably have different shape/size heads.
Build quality and durability are probably easier to get a generic view of, but an “affordable” price is another very subjective constraint. Where I draw the line is probably going to be higher than some would, and for sure, lower than others would. Whatever the “affordable” price of my theoretical ideal headphone is couldn’t be too much more expensive than the next best, given that many of the units available now are still pretty damn good.
I doubt it’ll be overly controversial to point out that for me, the order of those criteria are probably going to be comfort and durability in joint first, Sound quality (to MY ears) after those are both met, and price is within what I would class as reasonable.
And while I’m dreaming, a cure for cancer, an end to poverty, peace over the whole world and a large bowl of strawberries and cream, please. 
Yes, the only one I expect to get is the latter.