The best sound that I reminisce was the PlayStation 2 startup[0].
The sound was oddly terrifying as a child, and you booted it up in the dark. But I guess the nostalgia makes me miss it.
In addition, Windows XP and Windows 7 were the two most familiar Operating Systems that I used, and their startups probably are the most pleasing to my ears. [1][2]
[0]https://www.youtube.com/watch?v=fZet5jQmY1o