If you only wanted or needed 1 Dutch oven, it should be a #8 , which is also the most common size, at least in the Griswold choices. On the other hand, I have #6-10, but when cooking for a group, I use the #9. I'm keeping the #10, just because I like it. Another item getting scarce are the Griswold trivets in each of the sizes.
Trivets could add as much as $100 to the price of a Dutch oven. Antique cast iron cookware is one of the few antiques that when used, doesn't affect their value.

Pap