Ohhhhh… Definitive.
“My Name is John Wellington Wells” is my favorite song from the G&S canon. It is an excellent patter song, as well as being completely solo for added difficulty in singing. (“I am the Very Model of a Modern Major-General” is a cakewalk in comparison, because of all the rests the Major-General gets while the chorus sings.)
Although, “When I Was a Lad” from H.M.S. Pinafore is also one of my favorites, just because of how nicely cynical it is.