Best: unquestionably Paul McCartney. Saw him perform in Seattle about 13 years ago and ended his show with remaining members of Nirvana joining him onstage and jamming.
Honorable mention: too numerous to mention all but stand outs include Steve Winwood, Buddy Guy, Robert Cray, Lucinda Williams, Allison Krause, Karla Bonoff, Spyrogyra, Simon and Garfunkel, Leonard Cohen, Farm Aid, and my first concert experience ever, Humble Pie :)
Worst: Sting (not Police)
Up Next: Eric Clapton