I can think of maybe around 10 single DD universals that can contend for the top spot.
Heard none of them personally (except donguri), but i've read some opinions:
Faudio Major
Sony EX1000
DITA Truth/Dream - which one ?
Senn IE800/s
JVC FW10000
Beyerdynamic Xelento
Campfire Atlas (or Vega maybe?)
JVC FD1/X
Hifiman RE2000
Donguri Keyaki Ti Plus
Which ones can we add ?
Which one of these IEMs can we give the boot ?
We can maybe go about a process of elimination until we arrive at the winner, if there could ever be one
This might turn out a fun exercise!
I have:
Sony EX800ST
JVC FD02 modded (FDX1 ordered)
A&K T8iE MKII
and tested the JVC FW10000 a bit, but not too much because they are simply outside my budget.
Listening mostly to FLAC files with the CHORD Mojo
Out of those 3 I like the A&K T8iE MKII best, soundstage precision and resolution are amazing, extension is very good, but their signature doesn't emphasize mids and vocals as much as the FD02.
The FD02 is really good value at $220 in JP, very smooth, great sub bass, silky smooth treble and I love the vocals. Most emotional I have heard.
EX800ST is the "honest", slightly drier and more neutral than both others. I wish I'd listened to the EX1000 when I had a chance though....)
The FW10000 impressed me a lot with the classical music I tested it with, it sounded incredibly close to a live concert. Very transparent and open from what I remember.
Honestly all good IMO, each different in character, but all very good. But once you consider costs, the JVC FDX1 with 3 different nozzles with different tuning looks like the best value.
I paid $550 for the used T8iE MKII in mint condition, which again is excellent value IMO. But I'd not pay the $1200 list price or $1500 for an FW10000.
And another thing is fit and comfort.
The T8iE MKII with Spinfit tips are fitting me very well, with good seal, secure fit and high comfort.
The FD02 with Spiral Dot tips on the other hand sound very good but they just don't seem to fit my ears that well, and the left ear piece never sits securely.
This really has a big influence on their sound, and on how much I listen to them.
(I have Spinfit CP500 on order for the JVC and hope they will fic that)
Bottomline: All of the above are very good IEMs, but sound signature, fit and even value are a very personal thing.
PS: Never heard them, but during my searches I read several times that the Sennheiser IE800 are kind of outdated or surpassed.