I wouldn't say it's my favorite, but the old Apple floppy drives were pretty nice. (You could tell if your program was loading properly by listening, a skill I used right up until hard drives got too quiet.) I remember being jealous of my friend whose Apple //c drive sounded nicer than my Apple ][e :)

I think the sounds we remember most strongly are the ones that played when we were anticipating... modem sounds, floppy loading sounds, the part of vinyl records before the music, etc.