8 p.m. on O 60 The Next Best Thing: Who Is the Greatest Celebrity Impersonator?Can you sing like Celine? Move your pelvis like Elvis? Moonwalk like Michael? Declaim like Dubya? Then why aren't you on this show? Oh, well, maybe next season — if there is one. Host Michele Merkin presides as amateurs who passed muster at auditions in four cities try to make a good impression — pun intended — on the judges and the home audience. The winner, determined by viewer votes, gets $100,000.ft r\ m nn ra