I recently had the pleasure of installing OS9 on a classic 2001 iMac – the "lampshade" one, though I don't think that nickname does it justice!

I was, and am, blown away by how responsive it is. Plus, the UI sound effects add to the experience in a great way. You can HEAR when you've done something "in the computer." Just a bunch of clicks and boops. It's fantastic. Makes you think about what we've just... gotten used to.