My vote was to the hypothetical regular Joe scenario, in which no matter what you did to him he will never be a pro, much less a prolific winner. Equally abstract is the implication in the question that we are considering the doping of today, tomorrow and the next million tomorrows. It is impossible to predict what medical technology will bring but I still have a hard time believing that an average athlete could be transformed into a top tier pro by science alone.
In response to the question of whether doping or talent is more important within the confines of the 0.001% of the elite pro ranks I would say that the answer is definitely maybe. Within the super elite of say the top 10 finishers at a grand tour, an event generally long enough to sort out luck and reward the fittest and best racers I would say that yes, if they were all clean, then the 10th placed finisher began doping and the event was re run he would win. From what I have learned about doping the effect is not only increased strength and endurance, but also better recovery - obviously important in grand tours.
The other factor to consider is muscle makeup. A sprinter will always be better at sprinting than say, climbing and vice versa. If in a hypothetically fully clean peloton doping up Cavendish and having him train for the overall classification will probably not produce a win, regardless of the outside help he receives.
So, I guess my answer is based on the circumstance. At the top end of a specific field a doper will beat a clean guy as long as they are of similar natural ability. I do not know the maximum delta of talent, though. But, I do not think that it is possible to change a person's natural physiology to the point that you could make a punchy sprinter into a great time trialist.
There you go Hitch, you have successfully goaded me into a reply I've actually put thought into for the first time on this forum.