What switching method guarantees the best QoS?

Using traditional telephony like PSTN is highly stable at this point in time. The downfall is costs through carriers or providers. Often companies can guarantee QoS in the 99.9999%+ range using PSTN.

DVP or direct voice peering is becoming more popular as carriers have developed ways to utilize things like the MPLS network. Also very stable and less costly provided it's used in a mass environment, otherwise, stick with PSTN.