It is all down to opinion. Quite frankly this list are not worth printing. What criterias do they base these judgments on. It is all objective and down to individual taste and genre preferences. One man's meat is another man's poison. Whilst some of the people on the list are legendary artists a few cannot be called great singers by any stretch of the imagination. Bob Dylan is a great example. Whilst he is probably one of the most popular artists of the twentieth century he aint no singer technically. He has poor technique, tone and frequent pitch problems. Many people, myself included, cannot bear to hear his whiny out of tune voice and would not have him featured in the top 1000. This whole exercise is a waste of time. Ask 1000 people for a top 100 and you get 1000 completely different answers. One thing that also bothers me about such lists is that current pop artists are usually rated higher than all time greats. The current pop artists are generally forgotten and plunge into obscurity in six months time.

http://www.soundclick.com/bands/songInfo.cfm?bandID=687727&songID=5238055