The first answer is so professional! If you are asking about mobile phones and tablets, the obvious choice is the "Android" operating system. If you ask about computers, the mainstream ones now are win7 and win8, win8 is too high-end. The mainstream is win7. Please note that win7 has a difference between 32-bit and 64-bit. If the memory is greater than 4GB, please choose the 64-bit flagship version, because 32-bit only supports a maximum of 3.25GB. There is also an older system called winXP, but in today's fast-changing world, the stability and compatibility of XP are no longer as good as win7. Understood?