Not many spring to mind really - Vincent Cavanagh was amazing when I saw Anathema supporting Opeth last year, but there wasn't any particular MOMENT.
Chino Moreno of Deftones has all my favourite individual moments, basically because of of his awesome powerful screams. The one at the end of 'Royal' from Diamond Eyes stands out. My friend and I loved that song and the scream, so we were really hoping to hear it, but we had no idea if: 1. They'd even play the song and 2. What sort of form his vocals would be in, as it was the first show of the UK tour. Thankfully they did play it and he nailed it
I recorded the end of the song especially for the scream
https://www.youtube.com/watch?v=aizKkxhBQNsOne other thing I thought of - although it wasn't actually a live gig I was at - rather it was the guys from Bad Salad/VRA the DT tribute band doing a live video stream of them rehearsing for some shows. They were playing Octavarium and the final "Trapped inside this Octa-va-ri-UUUUUMMM" from their singer was so powerful and unexpected.
Here's a clip from one of their shows that will give you an idea (20.16 if the video doesn't automatically skip there)