"Downtown" from Little Shop of Horrors. When I saw it on tour this song blew the roof off the house. That last note always gives me chills.
"Out Tonight" from RENT. Though not "classic" I think this is the only song in the show that really rocks out and when Mimi ends in that pose facing Roger it's like you have to catch your breath.
"Petrified" from Taboo. If you know the song, this needs no explaining.
And "The Time Warp" from Rocky Horror. I was in this show a couple months ago and every night when we finished that song you could feel the electricity in the theatre.
"Who says you can't bend over backwards and eat bugs if you want to? I guess the bugs would probably say you can't do that that, but assuming that they are willing and consenting bugs, then there's no problem. Let's wig out eating bugs."
-RuPaul