I have been reading through this discussion and it's interesting to hear all the responses but I still don't see much in answering the original question so I tried a search myself and came up with some pretty interesting information.
Mind you, it still doesn't answer the question but it sure gives a lot of insite as to what might have been the most bought pattern.
here's an interesting webs site that talks about the old quilts
Both of these links takes you to articles that are very interesting
http://hartcottagequilts.com/his9.htm
http://hartcottagequilts.com/his11.htm
SO far as I can tell, it might have been the Sunbonnet series.But when you talk about patterns does that also includes embroidery patterns? if so then check out this link.
http://hartcottagequilts.com/his5.htm#lac
it seems there is a huge difference between the oldest patterns used and the oldest patterns sold