Are we counting festivals here? Because in that case it's too many to count.
My favorite concert I've been to was Ninja Sex Party, with TWRP and Planet Booty warming up for them. TWRP is my favorite band, and Ninja Sex Party is probably my second favorite, so that was an absolutely magical evening.
Another one of note is when I saw a band called Darkened Nocturn Slaughtercult. It's not a band I had listened to before, and I haven't really listened to them since. I don't care too much for their music to be honest. What made it special is that this was at a very small festival in Italy that me and my band were also playing at. We had already played, I was quite drunk and hopped up on adrenaline from playing, and during their set, the singer of DNS would drink pig blood and spit it at the crowd. I was in the front row and got absolutely covered in blood. This is not normally my kind of thing, but in that moment, it was the perfect end of an awesome night.