I don't think I name only one...
When you say "celebrity" that leaves a wide open door. The word "celebrity" could encompass anyone that has ever been considered newsworthy, whether or not they ever accomplished anything really worth remembering. Even politicians could be considered celebrities, or notorious criminals such as some who were executed for unspeakably heinous murders, or wealthy persons who may or may not have been generous with their money and/or time. Then there are people such as "Octo-Mom" and Paris Hilton, the publicity seeking lawyers such as Gloria Allred.
For the purpose of this Plinky Prompt, I will confine my list to the acting genre.
I really don't think I could honestly say I only have one favorite celebrity. I enjoy watching many of the older actors and actresses, many of which are no longer living, such as Jimmy Stewart, Doris Day, Julie Andrews, Myrna Loy, Red Skelton, Roy Rogers Cary Grant, Lynn Redgrave, Jane Russell, Ingrid Bergman, literally too many to list any more.