I think that question hasn't really been fully answered yet. But when you read up a lot, as I've tried to, you find worrying snippets like this, in the glowing review of the Olive 4HD, which costs over £2000.
"And comparing the CD quality with that of Marantz's excellent
CD6003 was interesting, too: there wasn't much in it, with the 4HD proving, at the least, an attentively rhythmic and expressive listen."
Now, the Olive does have huge hard discs, so is a 'server' not just streamer, and has an appealingly unique design. But to be about as good as a £300 CD player.... for more than two grand!
I'm still looking for a proper assesment of outright sound quality for these streamers, both on CD (or equivalent) and tuner, when used as a source in a good stereo system. So that's FM versus internet radio, and CD -v- its ripped or downloaded version. Currently, my best guess is that price is a fairly good predictor. So you might get something like this, with the best first:-
1. Naim NDX
2. Musical Fidelity CLiC
3 = Marantz NA7004 and NAD C446
4. Cambridge NP30
Cyrus Streamline and Naim UnitiQute in the middle somewhere, because they include amplifiers.
There will be fans of the Logitech Touch with DACs, and many other permutations, like the brilliant little Marantz M-CR603 (including amps). I realise there are many other devices out there too.
I hope others can suggest a better list!