I have heard, in chrono order over the years:
H2O 250 sig (early model - and it's the only one I haven't actually owned from this list).
Spectron M2
RedWine Sig 30
Nuforce Ref9 V2
Bel Canto S300
PS Audio A100
NHT Power2
Channel Islands D200
All have come and gone (but the H2O as it was never mine). Some lasted less than others, but the max was about 2 months. I'd put the CI D200 at the top. The H2O was good but it was a long time ago and I was less experienced then so I can't say for sure what I think of it.
Oh - I missed one - the Lyngdorf TDAI2200. I still own it (albeit it's been only 3 months) but I have stopped looking for amps, whereas it used to be that I'd be constantly on the prowl for a new amp. This is the most natural-sounding amp (well, it's more than an amp) and I NEVER find it compressing at loud volumes, or missing frequencies at low volumes, or sounding anything but 'just right' all the time. It reveals recordings quite well but not ruthlessly so; it does warm up the sound a *wee* bit. Just enough IMO.
For perspective: Have also owned a variety of budget to mid-level tube amps, mostly integrateds. No can do any more - if I ever went tubes again it would have to be on the high end, where I am given to understand they sound more like very good solid-state although I have not been privileged enough to hear any but the VTL Siegfrieds at a dealer demo once. Very nice but at $40K it better be.