Probably it's because it is the best material she has worked with but "Show Off" is probably her best performance of the three. Of course she hits "Gimme, Gimme" out of the park, but the song is not nearly as strong as "Show Off" which is incredibly witty and she handles it like a pro (and she has proved to be the ultimate performer of the song with her two replacements getting lukewarm reception).
Don't go for "Astonishing," she was terribly misdirected and the song can't compare to Jeanine Tesori's fun tunes in MILLIE or the incredibly witty lyrics of "Show Off."
"Some people can thrive and bloom living life in a living room, that's perfect for some people of one hundred and five. But I at least gotta try, when I think of all the sights that I gotta see, all the places I gotta play, all the things that I gotta be at"