(apologies if the following is pedantic or already familiar)
It seems to me that there is no such thing. 'Neutral' encompasses a pretty wide swath of sounds depending on who you ask, and in practical use 'reference' appears to be rather individuated as well (probably especially with IEMs due to the fit/anatomy's effect on their sound). This even shows up among reviewers who have heard a lot of the same gear, they tend to have their own distinct selections for what they most frequentlly make comparisons with.
There are a some "schools of thought" that take shape in ranking threads like Crinacle's grading thread linked above, but these too are ultimately based on one person's taste and methodology or taste in methodology (see also Vitu Fortuna's IEM SCORE CHART, Jelt's Fit For a Bat, Flinkeniks' Ranking the Stars,ShotgunShane's iem thread, ljokerl's ridiculously massive Multi-IEM Review thread/databse, etc....).
I think there are also popularity or history-based "reference cults" that form around particular headphones. The Andromeda seems to fit this bill currently, as the UERM once did (the current version UERR seems to be regarded differently). The ER4 has been around seemingly forever in various forms. I probably belong to the Cult of ex1000 (excommunicated after the reformation, condemned as a disreputable warren of Sony fanboys with knives in their ears).
Another approach is the quasi-objective 'target curve' approach. There are a few IEMs that explicitly strive for frequency responses developed through scientific research- most notably the Diffuse Field, Etymotic and Harman curves. Off the top of my head the Etymotic ER4S/SR based on their research, and AKG recently released the N5005 which supposedly has been developed in conjunction with their parent company Harman's research.
For my ideals, most likely answer I can think of is ER4- not the 'ultimate' 'clear, detailed' iem (though it is clear and detailed),but due to longevity, consistency, and relatively moderate cost, probably as close as it gets to 'universal'.