[oldjudge]

For me the top two are Old Judge and T206. I don't know what the third set would be--probably Goudey, but an argument can be made for E107, W600, M101-4/5, E135 too.
In my mind, Old Judge is the best because of the scope of the set as well as the fact that it was the first major baseball card set. Interestingly, it has 523 different subjects which puts it close in numbers to T206. However, when you start comparing apples to apples (total different poses) it is much bigger. The only argument against Old Judge in my mind is that it is not really a set, or even really a baseball card set. It is composed of multiple sets, many of which involved cards other than baseball players. Calling Old Judgea set is like calling Topps a set without differentiatiing by year.