I've been happy with the ES2 on the portable front for quite some time now. I also like the SR-001, soon to be a heavily modded SR-001 on the transportable front as well, though "like" doesn't equal "completely happy with," since while I like the sound signature, the technical merits could be better.
On the stationary front, though, things are a bit different. Or rather, were a bit different until I found the O2 Mk1. I could maybe become dissatisfied with it in a few weeks, but for the time being I have no complaints, and the areas that could use improvements are all areas in which a bigger amp (using a transformer box and a speaker amp ATM) will bring about improvements.
Prior to this, though, I was never really happy with anything. The (bass-light) K340 had a lovely sound signature but the comfort was just appalling; the right earcup had to be held in place by hand to get a good seal in every pair I've tried. The HD650 sounded amazing in push-pull mode off my speaker amp but there were issues with headroom. The HD600 was nicely neutral with great tone, but it wasn't fast enough or detailed enough to musically disappear. And the SR-404, while technically great, had a nasty midrange coloration and precisely the wrong sound signature.
I realize that after I heard the HE90, I was subconsciously seeking the same sound in my rigs. That's not a very realistic expectation since very little sounds like an HE90 short of the HE90. The balanced HD650 comes closest but it's still not the real thing.
However, the O2 is on the same level as the HE90, with admittedly a different sound signature, but one which still pushes all the right buttons for me. I guess it took listening to a comparable-quality phone for the shock of the HE90 to fade.
Well, it took only damn near three years. About time.
I guess that the O2 is here to stay, but the rest of the signal path is going to change, plenty and often.