Joe Williams was wonderful but not at all like Nat King Cole, who was also wonderful, IMO of course. To begin with, the two sang in different vocal ranges. Joe was a baritone and sang a lot of blues tunes, some of them quite obscure. His improvisations were unpredictable at times. Nat was a tenor. He was also an accomplished jazz pianist and accompanied himself on his earliest recordings, but his work was very mainstream for his time. He took fewer chances than Williams. Hartman was in my opinion the greatest for purity of tone, his work is more in between Nat and Joe, in his choice of material. I think I have all of Hartman's recordings. I wrote in another thread that my go to LP for comparing systems and evaluating upgrades is Hartman's "Easy Living". Human voice is, for me, a good constant.
Pablo recordings were typically of very high quality in terms of SQ. My only quibble is that they came along more or less after many of the great artists on that label had reached and passed their peak capability. Basie and Ellington when they no longer had a stable big band to lead. Ella when she had really lost that purity of tone for which she is revered. Sarah Vaughn was still in good voice when she recorded for Pablo; that is one exception. Joe Pass and Milt Jackson are two others. This is not to say that those Pablo LPs are not anyways quite enjoyable.