OK, Doc, you've got a good start - next you need American Beauty and/or Workingman's Dead, then Europe '72. The first 2 are the the boys finally figuring out the studio, and are chock full of exquisitely crafted songs, and the the last combines that with the improvizational majesty of Live/Dead - and the vocals on all 3 are much better that L/D; AB & WMD due to care in the studio, E '72 I guess they were just on those nights (I have concert tapes of other shows from that tour that don't sound that good) - and on E '72 you get 5-part harmony! But, even at their best, nobody will mistake them for trained voices with perfect pitch (well, I think Lesh has it, but you wouldn't know from his singing...) Objectively, can I say any of them sing as well as, say, Peter Cetera or Brad Delp? No. Would I listen to Cetera or Delp instaed of them? Only if you chained me down and forced me. I can't explain it, but I enjoy their singing, such as it is.
Peter