There have been a ton, hard to narrow it down, but a top 3 at a thought could be:

Steven Wilson - Mandela Hall, Belfast, 2018

I saw Porcupine Tree in 2010 during their touring of The Incident. It wasn't a great gig. Maybe because they knew they were going to split, maybe because I didn't like The Incident and how many songs they played off it. 

But this Steven Wilson gig was something else. He was, in my opinion, at the height of his solo career. On top of that he played a bunch of Porcupine Tree songs. The whole gig was around 2.5-3 hours long, it was incredible.

Shining (Norway) with support from Caligula's Horse, Raketkanon, and Jack Dalton - Camden Underworld, London, 2015.

I didn't care much for Jack Dalton, don't listen to them, but this was Caligula's Horse before they become known, their first gigs outside of Australia. They were raw and incredible, giving it their all.

Raketkanon, who I didn't know, tore up the stage in an incredible set that I still remember to this day.

Shining were at the height of their Blackjazz era, with a frontman who effortlessly switched between guitar, singing, and saxophone solos. The whole gig start to finish was endless energy, I left buzzing.

Dream Theater and Opeth co-headline with support from Big Elf and Unexpect, Wembley Arena, London, 2009.

Two prog giants in one gig, both playing full sets, was unreal. Opeth were touring off the back of Watershed, an incredible album, and Dream Theater were still going strong even if their albums were a little samey at this point.

I knew nothing about Big Elf but thought they did a great set. Unexpect were chaotic, but I expected that (it was not unexpected). Wasn't sure how their music would translate to live and, well, it didn't. They needed a much better setup. Still, it was a band who only ever did 3 gigs total in the UK so I'm happy I got to see them.

In fairness, I love this gig for a second reason, and that's for what it brought about. This tour was called Progressive Nation, organised by Mike Portnoy (Dream Theater's drummer). They did a few incredible lineup tours in 2008/09 before deciding to do what was my favourite music event anywhere in the world: Progressive Nation at Sea 2014. It almost ruined festivals for me because I can never find a lineup again as good where I liked nearly all the bands.