These two are my favourites so far, though I’ve yet to see them all.
Laser + mirror + sound
A laser shining on a mirror driven by a speaker creates cool patterns.
Interaural time difference and how to find your phone instantly
The science of sound localization is really interesting. This video is specifically about interaural time difference and how mobile phone ringtones are badly designed for the way our brains detect sound direction. You can make you phone easier to find by changing the ringtone to white noise. Also, emergency vehicle sirens like ambulances are badly designed for the same reason.