There is no correct answer to this. It depends entirely on what you're trying to do with the computer. First, you must figure out what applications you want the computer to run, then you'll need to determine which operating systems those applications can run on. Do some research on the web, and you can find out which operating systems are most suitable for specific applications.

As Bucky rightly said "There is no correct answer to this". Apart form the results differing form application to application - some apps and some O/S will only run on some computers. For example HP-UX will only run on computers with PA-RISC cpus and maybe a few rare others, whereas Solaris will run on Sparc, Intel and AMD. If we look at PCs there are 4 main operating systems you can run now. Windows, MAC OS, Solaris and Linux. MAC OS is limited to Mac computers - its a really nice OS - seems quite fast to me - but difficult for me to comment on as I havent used it. Windows and Linux to complicate things each have many flavours (Windows:- XP, XP Pro, NT, 2003.. LINUX - Red Hat, Suse) and some of these falvours have flavours themselves. From benchmarks I have seen, Solaris 10 is faster than Linux on AMD Opteron, but Linux is slightly faster on 32 bit computers. (Solaris 8 an 9 were way slower than linux). However you cant run Linux on a 72 processor system and get it to scale. So 1 CPU 32 bit - Linux wins, but go above 4 CPUs and ... Remember there are Lies, Damned Lies, Statistics and Benchmarks.... Speed is really only one factor to consider- what really matters are:- Usability, Applications, Security, Availability, Support or 3rd party hardware, Support, Cost In my opinion (but Im sure others may disagree):- For usability, applications, support for 3rd party h/w - Windows wins. For Security availability and cost Solaris 10 wins Linux for me always comes second in all these categories. Thats not a bad thing, becuase the other two are always 1st or 3rd. Security will always be a major problem for Windows. However the user interface for windows is way better than Solaris... Dont forget MAC OS - I cant score it cos I dont know it so well - but it is much loved - and Im sure would score well in several of these categories

My answer for this question is LINUX which is faster and safer.

Seems to me MS-DOS 6.1 is fastest. It is NOT multitasking, so your program written in assembly language will have the entire use of the CPU, with no overhead at all.