Probably the Metallica-hosted Summer Sanitarium tour. I saw the one in Dallas, when Hetfield was out because of a skiing injury or whatever, but the sheer number of kickass acts made it a blast anyway. I think it was System of a Down, Powerman 5000, Kid Rock, Korn, and of course Metallica. One thing that made that concert a bit special was that all the backup acts came out and took turns filling in for Hetfield either on guitar or singing during Metallica's set.
Second after that would have to be Tool.
One concert that I surprisingly hated was Widespread Panic. I really figured it would be good given their rabid fanbase at the time, but that kind of music just isn't for me I guess.