I think nothing comes close to windows 95 startup sound: https://www.youtube.com/watch?v=miZHa7ZC6Z0 It was good to hear it. Considering stability of the systems of those days, listening this means that the machine was able to boot and in good enough health to load and play the startup sound.SGI Indy bootup may sound cool but probably annoying after the first few times you hear it: https://www.youtube.com/watch?v=CH9saUP2460
Another I find cool is this one from apple: https://youtu.be/d8szceStqZI?t=37