Bon Jovi at MSG a few years back. Got a cheap ticket at the last minute figuring with all his hits that the show would be a can't miss. Welp, it did. Lots of stuff off his crappy recent stuff and his big hits all sounded flat as a pancake with no energy at all and were just boring. Honestly when I saw this topic and thought about it there were very few shows I have ever been that I didn't enjoy.
But as a reverse of the topic and a show I wasn't expecting much out of and ended up enjoying greatly: Kid Rock at Pru Center. Got a really cheap ticket because I just wanted to go out and he had a few songs that were big hits even though I was never a big fan. And he put on like a 90 minute show that just totally kicked ass and was entertaining as all hell from both a music and theatrical standpoint. Seriously he is a great live performer.
I got dragged to a Kid Rock show here in Winnipeg. Didn't think much of him before going to be honest but it was a hell of a show. Turned out I liked a fair bit of his music and he was remarkably inoffensive, total opposite of what I was expecting.
Opening acts are often surprisingly good and on many occasions I've enjoyed them more than the main act.
GnR opened for Black Crowes at one show - had no idea who they were but I didn't forget after the show
Aforementioned Kick Axe/Whitesnake prior to Quiet Riot
Shinedown before Kiss - Kiss was pretty damn pathetic that show, literally going through routine motions. Shinedown blew them out of the water and I knew nothing about them before the show.
Then there were these pair of shows I saw in Minneapolis on back to back nights a few years ago. Went down to see Tom Petty (huge, huge Petty fan). I'd seen the opener previously but they were really good that night. Little band called Pearl Jam? You've probably heard of them. Next night this group called the Heartbreakers opened up for Pearl Jam. Both shows were hugely disappointing.
I also saw Petty on his last tour in Minneapolis. Opening act was Joe Walsh, which was neat because I've never seen the Eagles live. He did about 90 minutes himself and then as was typical with Petty shows, a couple with Petty and the Heartbreakers as well. We pretty much got two full shows that night. Only time I've seen Petty not do a tune or two with the opening act was when Joe Cocker opened for him at Bell MTS back in maybe 2014 or something like that. It was between the Winwood shows.