I know we look like we don't want to answer you, but there is no answer because there isn't enough data.
to me it mainly depends on the headphone. with the proper headphone specs, even a cellphone will measure real great. the fun starts when we go into hard to drive stuff, and really easy to drive stuff. both ends are troublesome for the amp section of a DAP and stuff optimized for one end might suck at the other end. so the perfect DAP is most likely BS unless it has a really great amp section. something that strangely enough, some of the most expensive DAPs don't have(ZX2 anyone?).
add to that your own needs for screen size, weight, functions, and most likely you'll end up like me with no clear choice because nothing does all that you want.
I like the sound quality of the X1, and into my amp it's really a great source. but the functionalities...
I like my sony A15 because it's small and I charge it maybe every 10days or so, using it is cool really, but 4ohm and some hiss make it hard for a few of my IEMs(and overall specs aren't amazing). and TBH I preferred the UI of my A867, except that the battery sucks. and those are the DAPs I really like and enjoy using...
I've tried a good deal of DAPs that are a nightmare to use, bugs, no screen, limited file format, super loud hiss, strong roll off in the bass or the trebles on IEMs, did you try the touchscreen on the dx50 when it came out? did you try carrying a colorfly CK4 or a studioV or a X3 near a cellphone?
looking for the right DAP is a battle for me. and we all have our own preferences, that makes it even less likely to be able to suggest a proper DAP to somebody else.
also I dislike both android and apple for mostly nn audio reasons, so that's a great limit for me.
in the end I use the sony A15 with some etyKid IEMs most of the time ^_^. that's how I roll when I have a pair of jh13 that are still my favorite IEM once EQed and played into my amp. but I'm at a point where good portable sound is a bother more than a pleasure. and the only reason why I'm not using one of my sansa clips, is the battery life of the A15. else rockboxed stuff are really the best.
that's not what you're asking, but I don't have a better answer, measurements just aren't available and the proper amp is a better answer to sound quality than proper DAP.