The guest hosts of NBC's comedy sketch host can help make or break and episode, so let's celebrate the ones who brought their A-game.

Oscar-winning actress Scarlett Johansson guest hosted the milestone 50th season of NBC's sketch-comedy series, Saturday Night Live, this weekend. Her opening monologue turned into a cast sing-a-long about the ups and downs of working on the show -- where Johansson's husband Colin Jost is a writer and "Weekend Update" co-anchor -- set to the tune of Billy Joel's classic song,