Though it's a good question, which I have been thinking about before, there's one thing that made me conclude: BeOS. As tiny as 10 megs (a 500 megs image, where the rest of the megs are being used as BeOS formatted disk space, which could be ran from Windows), compared to the 300 megs of Windows 98 at that time, and it had a HUGE functionality, it even supported my tv card as standard, back then. It was super fast too. I loved that OS to bits.
Pity it's today obsolete, not supporting my usb keyboard, usb harddisks and other hardware. But back then, everything worked out of the box :-D
Just wiki'd it, and I saw it has been made by former apple developers. And it has been adopted by Palm. Another manufacturer whom's OS I love :-)
