Neither a “best” nor a “worst” moment but very definitely a moment that had me questioning the veracity of small kittens.
Also, laughing.
Mine had to be when the last remaining goon fired off their Anti-Gravity Device team up power. My only countdown tile on the board was Luke Cage’s Jab, Jab Cross so it added 2 turns to that, and the goon subsequently died a fiery death at Dr. Strange’s retaliatory yellow power.
These goofy moments from the AI make the “let me go make a sandwich while this cascade finishes” beat downs they hand out from time to time easier to swallow!
From the same node as the OP, when Osborn is trying to convince the Sentry that he’s really the good guy, and promptly says “Go, my Dark Avengers”
Good work, Osborn. No one’s ever going to figure out that you’re evil now 
This happened to me with Strang3’s Crimson Bands countdown!
Similarly, I had a Lieutenant with ~750 HP remaining place a yellow “Morale Boost” healing tile, causing Dr. Strange to Flame him to death.
“Man, I’m hurt… better try to heal myseOH MY GOD IT BURNS!!”