Are you talking about a new Mac? If so, then it will come with OSX Mountain Lion. As for which one is best, you are the only one who can decide that. Have you taken a trip to the Apple Store and tried out the machines that fit your price range? Have you talked to the Apple sales team in the store and discussed what your needs are, how you will be using it, how portable you want it to be, etc.
You need to think about all of those things and write up a list of the most important functions that you need supported. Then go to the Apple Store, share your list (which would, of course, include "under $1000"), and then play with the Macs that best fit your criteria.
I am thinking about buying a mac book ...