Windows or Mac OS X depending on personal preferences and how you define popular.
Both Mac OS X and Windows are popular operating systems. They are popular because people like using them.
Mac-OS=Disk operating system==Linux OS==Windows OS==Unix OS=
The most popular operating systems that can be uses on a MacBook include: Mac OS X 10.4 (Tiger), Mac OS X 10.5 (Leopard), Mac OS X 10.6 (Snow Leopard - due soon), Windows XP, Windows Vista, Windows 7, and several varieties of Linux.
Windows Vista and Mac OS are types of computer operating systems.
Mac OS X 10.6 still beats Windows. Mac OS X 10.6 systems are more stable and secure than any Windows, and this has been proven. 10.6 still beats Windows in performance. It is also in my opinion more useable than Windows 7.
No , you cannot run OS X on a windows machine. Buy a mac
Windows XP is an example of an OS. Windows Vista is an example of an OS. Windows 7 is an example of an OS. Mac OS X is an example of an OS.
"Yes, you can run a Windows OS on a Mac notebook with the right software. There are programs such as Bootcamp that allow you to create a partition and install Windows OS."
Pretty much the only reason Windows is related to Mac OS is that they are both operating systems.
Windows Vista and Mac OS X are examples of computer operating systems.
A desktop uses the same OS as any other computer, Windows, Mac OS, or Linux. Also, I'd recommend Mac OS X if you're getting a new computer. Here's the scoop: if the computer has an  on it, it runs Mac OS. If not, it runs Windows or Linux.
It will depend what you are trying to achieve and how you define better. If software is written to use Microsoft's Windows system and is not available for Apple's Mac OS then Windows would be better. If you are using a Mac made by Apple then the Mac OS would probably be more suitable but you can always switch to Windows on a Mac if needed. If you are looking for a cheaper, virus free, trouble free computing experience then Mac OS would be your choice.