Be cautious of whom you ask for a letter of recommendation. The most important part of the recommendation is how the writer speaks of you. Usually the best person to ask is someone you have a good rapport with on a day to day basis. Although big names look great on letters, it is much less valuable if the writer can only speak about you in generalities and regurgitates your CV.
If you have the opportunity to work with a well known physician, it is in your best interest to make yourself available to him or her for an extended period of time.