Depends on what you mean by quality, how much you're willing to pay for it, and what's actually available.

If quality means low latency and low jitter, and you can get T1/E1 digital telephone service, circuit switched end to end, that's almost certainly the lowest practical latency and jitter. It'll cost real money and you might not like the 8000Hz 8-bit sampling. ISDN calling is pretty much the same thing, but you only get two voice lines instead of 24 or 25.

If that's not an option, but low latency is still important, a solid wired connection with g.711 SIP is pretty close, but packets are 20ms of samples, so that adds to your baseline latency. G.711 is basically the same codec as used for t1. And internet jitter is probably not zero, but if your connection is well provisioned it's often not that bad.

If you want a wider audio band, maybe try to get SIP with G.722.2 (AMR-WB), this is the same codec used for 'HD Voice', and I don't think it adds too much delay.