I really liked "The Impossible Game", because the idea is so easy yet they make it really hard. All you do is that jump but the creator makes the background go faster.
I also liked the "Webcam Piano" because it interacted with you. It also was a piano you control with your hands.
The Impossible Game- http://scratchmit.edu/projects/10318822/
The Webcam Piano- http://scratch.mit.edu/projects/14217349