As a player, it had to be playing Iceman and, after being stuck above ground while everyone else was getting to fight Wendigos and racists, the sentinels all finally woke up and, in one big area effect action, I spent a bunch of plot points and caused d12+ complications on fifteen sentinels all by myself, then ice-slid myself and the Madrox duplicate I was with down to everyone else just in time for us to be there when the big explosion went off. And the duplicate that was the only witness to my spectacular feat was blown up in the explosion.
As a watcher, it would have to be having my players' characters sent through time by Kang's minions. One ended up in a movie theater in 1984 Manhattan, one ended up in a Virginia town surrounded by people in colonial garb, one ended up in a jungle next to a T. Rex, and one ended up in a big fallout shelter filled with hideous mutants. The good part was revealing to them that they were actually *all* in 1984. One in Manhattan, one in Colonial Williamsburg, one in the Savage Land, and one in the Morlocks' tunnel.