IMHO, no-one has the 'best' voice because everyones' ears differ.

My personal favourites would be:-

Male - Stan Rogers, Jack Hudson, Bob Fox, Dave Fletcher, Allan Taylor.
Female - June Tabor, Joni Mitchell, Flossie Malavialle, Eddi Reader, Karen Matheson.

Whether they're the 'best' is a matter of opinion. I my opinion, they are. :0)