In the US, likely SkyWalker Sound (Wow you'd have to see to believe), Das Boot (James Guthrie's studio) and possibly Bob Ludwig's studio (though I've not been there, I've heard great things). Lookout Sound studio in Montana has invested heavily in getting great sound also.
In the UK, there is no studio that cares about great sound and gets it more than Astoria (David Gilmour's). Real World (Gabriel) is very lavish but their goals do not include ultimate fidelity. Mark Knopfler has put huge sums of money into building a state of the art studio in London.