I agree with Bill, I go for the less common ones before considering the Dorado rare. They come up in auctions often and if you have the money to buy one, you can have one.
So on that note, how about the Ritz Cracker Timex? Other ones I would consider rare are the earliest Monroe's and any very early non-round ones. I once sold a non-round tank style Timex from early 1950s for over $200!