best-sounding class D amp?

I don’t know about “warm” sounding class d amps, but a tube pre-amp used with a Class D amp can achieve that. You want the amp to have 60kohm or higher input impedance to work well with tube preamps which tend to have significantly higher output impedance. I do that with AUdio Research pre to Bel Canto Class D amp. Just a touch of warmth with this combo. 

Also have a similar Bel Canto c5i all digital class d integrated amp. More neutral, not warm at all.

best-sounding class D amp?

Look at PASS amps much better than classl d pass on class d maybe in 10 years it will get better.

Best one I’ve heard so far had been at our club audio meeting, the Belcanto Ref 600M monoblocks which use Hypex NC500 with I believe their own buffer, they were very good into a very high end two-way that used the top Raven ribbon tweeter "almost" livable for me.
Then we put them on a pair of B&W 803D3’s and they were not as well received, upper/mid highs were a touch hard with glare yet subdued at the same time, we concluded that the Ref600’s liked the pure resistive loading that the Raven Ribbon gave around a constant 6ohms, but not the more complex loading the B&W upper range had, because the same B&W’s sounded sweet as with a Pass Labs linear amp we also had there.

Cheers George

I haven't heard enough of class d amps to form an opinion but a friend of mind which i respect very much on the audiophile side recommeded highly Bel Canto amps .

ATI 5***. Use N-core with a proper linear power supply by a pro mfg who has been around for a while and who OEM's some very famous names.. A Theta Prometheus at a cut price

I’ve only heard NCore based amps NAD and Purifi based VTV , hey both sounded very good but in my subjective opinion I liked the Purifi based. I believe Nord, March, Apollon make these. The one I would really like to hear is the new NAD M33 based on Purifi.