This can be a very difficult question to answer. It doesn't follow that the best winning dog/bitch in the chosen field (whether work, agility, obedience, show, etc) is the best to breed from. Sometimes the litter brother/sister is the best choice. Really though, you should wait until it is mature enough to see how it ends up. A pup at 8 weeks that looks like it has real potential could look/act awful at 8 months, 18 months even. But at 2 1/2 years old could be a real cracker ;-) What is easier to see is if it is NOT a potential breeding animal. In breeds where markings are distinctive, or colour is important, these can be seen early on and the pup should not be chosen. If I'm looking at pups in the nest I tend to look at them between 6 and 8 weeks. Then pray! :-D I have a male in the house just now that I kept from a litter. He's a nice dog, but I really like one of his brothers. One that I rejected at 7 weeks

;-) That's life :-D :-D
Edited to add: How old is your bitch and do you do anything with her? ie do you show her, work her, or anything? If you do and she is doing well then I would go back to her breeder and ask for their opinion. Hopefully they won't be kennel blind and will be able to help you. ;-)