Stax SR-001 Mk II. This system costs $249 and beats an HD600 with a pretty good portable amp, which costs one hell of a lot more, and is a lot less portable besides. The system beats the ES2 straight out of the headphone jack, though the two sound different enough that it's somewhat hard to compare. But, they're more or less on the same plane, and the SR-001 sounds fuller and less fatiguing, while retaining all of the detail.
Why the hell is it that only 3 or so people on head-fi have this system? Beats me. It's an amazing deal for the money.
The SR-404 is also a bit underrated, but only because it's not that easy to build a complimenting system around it. The SR-404 together with tubes and the right source can sound very much like the HE90, mostly losing in soundstage but competing well in resolution and tonal balance. The SR-404 can sound damn near ideally balanced to my ears, and it's not even remotely lacking in bass in the right system, unless you are a basshead or want more bass impact than a stat can give you. For $450, it's an awfully good bargain and a really high-end can. You can put together a well-matched SR-404 system in under $2k, and it will sound amazing.
I would say that the AD2000 is a bit underrated as well. Many obsess over the A-T woodies since they look gorgeous, but A-T open cans have gotten very little attention.
The most uberly under-rated unknown unrecognized under-appreciated unjustly unloved system on head-fi ever of all time?
Stax SR-Lambda Pro + SRM-1 Mk II. $10k performance for $300 on eBay (if you can find one).