For some, the answer is obvious. It might be a son or daughter. However, for many the question requires serious thought. One needs to consider not just who would be the best person (or bank or trust company) at the current time, but who will be the best when the trustee actually has to function. That may or may not be the same person or institution.
For example if the trustee or executor is required to serve within the next 5 years it shall be my friend Bob; however, if it is more than 5 years, it shall be my daughter Jill.
Experienced Wills and Trusts attorneys are helpful in assisting with the selection of the backup trustee and executor.