Quick reply...
Seriously, the best operating system would be the one which is so small it can be saved on a floppy disk, and so light that the battery can last for months with usual (phone-clock) usage. At the same time, if user wished, he would be able to run a CPU-intensive application and discharge the battery very quickly - it's user's own choice. The fact is, I dislike the idea that OS should grow in size, memory and CPU consumption as hardware grows. I would be prefer for the hardware progress to create a mobile which allows human to travel through a network of caves (no solar recharging) for several days (weeks) and still have the mobile working (as lamp-torch, as map in absence of both Internet and GPS, as pedometer to manually create a map of the caves, as sonar if necessary). Theoretically, it's possible. Practically, almost any mobile OS is bloated, to an extent.
Best wishes.
_________________
Per aspera ad astra...