Wow, hard to choose when you have seen as many musicians as I have!
Best is Bruce Springsteen, seen him five times, and the first was the best, on The River tour. When I mention that gig, I always tell people that I took my then-girlfriend, now wife to see her first ever proper gig. I had seen thousands by then of course.
When we came out, she asked me if all gigs were as good as that. I told her she need never ever see another live performance for the rest of her life because she will never see anything better than that.
I have been to thousands more gigs since then, but it remains my best ever.
The worst was BB King in Montreal not long before he died. This was my chance to see a living blues legend - this guy picked cotton in the fields as a child.
He was beyond dreadful. He had simply lost it, maybe just old age, maybe senility, I don't know, but he played 'You Are My Sunshine' (!!) and made it last twenty five minutes!!! His band just looked embarrassed, people started to leave, and believe me, Montreal audiences are the most adoring and easy-to-please anywhere in the world - they do ovations when musicians come on before they've played a note!
I found out from some contacts who knew his 'people' that his brother, who acts as his tour manager and looks after him, had been trying to persuade to give up live shows, but BB just didn't want to let go.
It was tragic, but I felt really sorry for the guy in the next seat to mine, it was his first gig, he'd driven four hours to get there, and it was his ambition to see a legend. What a let-down.