Without any questions, Marillion opening for Rush in '86. I think they played the whole side one of Misplaced childhood.
Dream theater opening for Iron Maiden. I think it was in 2012. They played 5 or 6 songs and I was charmed. I bought my ticket for them. Did not care much for Maiden because I have never been a fan of pure metal.
Loverboy opening for April Wine in '81.