Best was Pearl Jam in 1991--just as their album "10" came out, only about a month or two before they were known to most people. After tickets were sold, the band found out that the venue on campus at Marquette University had fixed, theater-style seating. Concerned for the safety of the crowd, trying to do a mosh pit around those seats, they insisted that the show be moved to a different location--which was far smaller. Many tickets had to be refunded--but those that got in got to see the concert of a lifetime.
Second best was seeing Tesla. They did a superb 2+ hour accoustic set, then in the middle of "Love Song", switched from accoustic to electric, and did anther 2+ hour set that way. It was superb!
Other great ones include the Eagles, Extreme, Simon and Garfunkel, Huey Lewis, the BoDeans, and yes, even Richard Marx put on a great performance.