Depende sa application mo.
Wiindows is no good for real time apps because of its muti-tasking capability.
Pero OK sya for most data processing and graphics processing apps. Linux has a version that is optimized for real time apps.
DOS (MS-DOS, Datalight DOS, ROM DOS, DR DOS, X DOS, FreeDOS, etc) for real time apps is OK and remains popular.
Siyempre for cellphones me kanya kanyang preference din. Symbian, Windows CE, etc...
For parallel computing, the OS of Cray and other mainframes are quite powerful too.