Today is ‘favourite guest appearance’.
Every now and then, someone pops up in a show and everyone goes ‘aaaaaaah’. Bruce Willis turned up in Friends for several episodes, returning to his sitcom roots. Will Wheaton’s appearances as himself in The Big Bang Theory are always funny, and Professor Stephen Hawking also has also appeared in the show, as well as The Next Generation.
It’s TNG that wins this. Leonard Nimoy’s guest appearance as Spock for the two-parter ‘Reunification’ is a powerful link between two generations of Star Trek.