In my experience dealing with multiple clients, I would say this is subjective, what sounds human to you might not be true for someone else, My reccomendation is to have 6 samples of voices, various accents british, america, etc... and have the client select the one they like the most! Good luck mate! IMO, PlayHT and 11labs are the most human soundig voices, however once vapi allows the new 11labs voices, the 3.0 version, they will befinetly the kings of human like conversations! Cant wait for that to be integrated 😄