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.
I levitated from my couch when Ahmed Best appeared as Kelleran Beq in The Mandalorian… and nearly lost it when he put Grogu on Jar Jar’s ship.