I've been to more concerts than I can count. Country, rock, rap, classical, I've been to them. Most all of them have been really great experiences and its very rare that I've had any problems with anythere. The only time I've even come close to having problems with anyone was at a 18+ Everclear show in Louisville, KY when some high school kids snuck in and were acting like they owned the place but that didn't last long and it was still one of the most enjoyable concerts I've ever been to.
As for who some of the absolute best artists I've ever seen - Everclear (2 times and backstage 1), Smashing Pumpkins (4 times and backstage 1), U2 (3 times and met Bono before 1), Weird Al Yankovic (1 time and I've never laughed so hard in my life, backstage after the show he was really nice) and front row at a Pearl Jam concert (2 times, only one was front row, the other was 10th).
Honorable mention goes to KISS and Elton John. I don't really like their music that much but I do have to hand it to them that they really know how to work the crowd and put on a good show.
Absolute worst was Nelly. The stage was pathetic, I've been to country concerts with more bass, and all it the entire was was Karoke night with the original singer doing the songs. Even though I didn't pay to go to that concert I still felt ripped off because I'll never be able to get my time back that I wasted there. Just for the record, I do like his songs that you hear on the radio but seeing him live was horrible.
Dishonorable mention goes to Tim McGraw. Decent show and everything but the night I saw him the sound was horrible. If it weren't for that it would have been a decent show.