hi,
I have tried Isoclean, Oyaide MTB, Acoustic Revive RTP Ultimate, Ayre, Shunyata, Walker Velocitor, BPT PPT (Oyaide + ERS options), QRT Powerbar, Audience (top end model) one or two re-gen i cannot remember.
As a rule of thumb i personally prefer the non-active, non-caps, non re-gen types. I prefer correct timing/speed to ultimate blackness. So for me, no Isoclean, no Audience, no re-gens, no Shunyata. I own the QRT Sym Pro with the BBT PPT power strip (Oyaide + ERS options) and I've upgraded all my plugs to Oyaide incl carbon wall sockets. For a long while, it's the best bang for buck. I recently tried the Acoustic Revive RTP Ultimate and that thing is amazing. Superbly natural, effortless and digs up subtle details esp in the bass region never before heard. It's not overt and won't hit you in the face but over extended listening session, you won't want to give it back.
In terms of my preferred ones atm: BPT (value for money), Ayre (quieter but relatively speaking more shut in) then finally Acoustic Revive.
Walker is good but pricy and things have moved on. Forget the QRT power bar, v coloured. Audience is nice but colour (but in a nice way). Oyaide MTB - not bad but competition far better.