It comes down to how much you want the entire series versus the “best” books. Personally, I like the first ten (I didn’t like the last one, “John Carter of Mars,” which never felt like a “real” Barsoom book.
Some folks I talk to on various forums, however, swear that the series is best through the first three or first five books. Barnes & Noble has a nice omnibus edition of the first five books, in either cheap or fancy hardback. If you’re fine with that, the B&N choices are quite excellent.
If you search on e-bay you might find the old SFBC omnibus editions. There are 2-3 novels in each volume, and I think you have to find 5 volumes to get the whole series.
If you look for the paperbacks, I highly recommend finding the 1980 book series with the Michael Whelan covers. Those are the ones that give me the biggest feel for what Mars should look like.