I was going to mention the dwarves song too. And Viggo's in RoTK.
Madeline Kahn's 'I'm Tired' in Blazing Saddles.
'Day-o'/'Jump In The Line' in Beetlejuice
The waltz scene in Legend (Goldsmith version). Ridley really should do a musical.
The library dance in Breakfast Club.
Steve Martin & Bernadette Peter's 'Tonight You Belong To Me' in The Jerk. Love it when she pulls out the trumpet!
Tom Cruise's Tropic Thunder dance. Didn't see that coming.