Top 10, in no particular order (subject to change):

Bruce Springsteen
Paul Simon, in Central Park
Simon & Garfunkel (first reunion, 10 yrs ago)
Billy Joel
Elton John (wish it was together with BJ!)
R.E.M. (multiple times at college for free )
Rush (multiple times )
The Ramones (outside, for free )
U2
Metallica/GnR

Honorable Mention:

Replacements (they were too drunk to be Top 10
Projekt Revolution (Linkin Park, Korn, Snoop)
Lollapalooza '92 (Chili Peppers, Soundgarden, Pearl Jam, Ice Cube,...)

too many to remember...