They would all be equally successful - basically it doesn't matter how many results a combination might have. First you have a chance of getting a common, medium or rare dragon, then you have a chance of getting one of those dragons in the sub-category. I think rare dragons at something like a 1 or 2% chance? There are wikis or pages on this board that would explain it in more detail.
So when picking a combination, the best thing to do is to try pick a combination where the failure dragons either do not take a long time, or where the failure dragons might be dragons you still need for your collection.