Well I guess I obviously regret not going to every ProgPower since I became aware of the festival, but I never had a real shot at going to most of them anyway. I had seriously considered VII in '06 though, and in hindsight I really should've gone. I love Evergrey, Jorn, and Circle II Circle to death, and this was also the last breaths of Thunderstone and Vision Divine before both bands went down the toilet.
One big singular show that sticks in my mind is Stratovarius in NYC on 10/02/06. It was a rough week with two other shows (one being Maiden), I had seen them a year prior, so I figured I'd save the money. Turns out the set list was like 70% different and much better than what I had seen before, and well, we all know what's happened to the band since.
Oh, and another was Dream Theater's "Score" show. The thing that pains me the most is that my best friend went and actually had an extra ticket he was trying to get rid of. The trouble I've always had with Dream Theater is that I'm not a big enough fan to enjoy the set list rotation thing, so I didn't go. In hindsight the set list was phenomenal (Another Won, Afterlife, Raise the Knife, Six Degrees) and the show itself is brilliant, as the DVD shows.
If it were to happen again, I'd easily make the trip to Chicago PowerFest just to see Nocturnal Rites. In 2006 I was reluctant to do it and thus didn't, but having seen the set list and again, what has happened to the band since, I feel I was a total fool for not going. If this band EVER plays America again, I am there.
And lastly, I feel stupid for not seeing the Whitesnake '05 US tour. The DVD put out from that time period is fucking phenomenal and the set list is brilliant. Now they are short both Tommy Aldridge and Marco Mendoza, and despite their popularity have not given me another chance to see them.