For me it was Ezra Miller showing up in The Flash TV show a few years ago during their big “multiverse” crossover event.

On the big screen, gotta give it to the recent Flash movie. The poorly CGI’d Nic Cage was such a great fan service. Also I managed to square myself from all the media past premier, that G.C. showing up at the end was a pleasant surprise.

Hugh Jackman in X-Men First Class wasn’t totally unexpected, but it was worth a good chuckle.