So far, it’s a tossup between Progressive Nation tour 2008 and Opeth/Katatonia 2013.
Both included Opeth.
The former also featured Dream Theater, and introduced me to 3, a relative unknown prog band from Woodstock, NY. I was instantly in love. I was seven months pregnant during that show, and liked it so much we followed the tour to the next city the following night. Between the Buried and Me played as well, but made absolutely no impression.
The latter was just the two amazing bands. It was at a venue where my husband and I are furniture, I was right in front the entire show, the bartender was generous with the Bombay Sapphire, and almost all of my favorites were played, even though they are on albums that are about ten years old.
So, introduction to new awesomeness + general awesomeness vs. Nearly orgasmic music experience.