Many people agree it's Bon Jovi - that they make the best atmosphere, engage the crown in the best possible way and just provide at least 2 hours of musical nirvana.
I believe it's different for everyone. If you're not into the music the artist is playing, even the best performers will have troubles reaching you.
For me, Bob Dylan is the best I've heard live. True, he's flake, and never repeats his performance twice, and you go to his concert not knowing what you will get - sometimes he doesn't acknowledge the audience at all, sometimes he acts like they are a crucial part of him. I've seen him in both moods, and can say that when he plays and sings, there is no way you can stay unmoved, untouched...
Depends on how you measure "greatest". As in "your favorite", "most popular", "best-selling", "longest performer". If lip-syncing is omitted, then, for me either Mariah Carey or Celine Dion. The only two performers I've ever seen hit high notes the longest and the most frequent.