“Who Wants to Be a Millionaire,” starring Regis Philbin, was back at the top of its game last week, monopolizing the Nielsen ratings, with the Thursday, Tuesday and Sunday editions of the quiz show ranking as the most-watched programs of the week. NBC’s reality shows “Weakest Link,” “Fear Factor” and “Spy TV” took the top three spots for the week in the 18-49 age group demographic, though the second outing of “Spy TV” in the Thursday 8:30 p.m. slot was down from its premiere the previous week. The Top 10 for June 25 to July 1: