The all-time best was seeing Kid Rock back in '96 before he really went national, in a dive in Kalamazoo that I wasn't even old enough to get into. That was an awesome night, Kid Rock bought me a beer and I've still got the flyer he signed for me. And when he got up on stage he did his set, which was more rap than anything else back then and not really my kind of music, then sat down with an acoustic guitar and said he was going to play "until there wasn't a single MF left to listen". We were there well past closing time.
After that would have to be Kid Rock, Metallica, and Ted Nugent, New Years Eve '99/Y2K at the Silverdome (Detroit Lions old stadium). We were main floor right up near the stage, standing section, and it was wild! They all came out on stage together for the countdown, and played Detroit Rock City right at midnight.
And for the experience as much as the music, Woodstock '99 has to rank right up there too. Three days of almost no sleep, the road trip to get there and back, all of it was just a classic 20-something "freedom" experience.