It's taken the Billboard magazine rental chart three weeks to verify what the rest of the home video industry suspected on Labor Day weekend--that "Moonstruck" is by far the nation's most popular rental. This week it officially takes over the top spot, sending "Good Morning, Vietnam" tumbling to third. As of last week, Cher had two movies in the Top Five. Her "Suspect" (No. 4) is still a hot item after two months on the chart. James Woods, an expert at playing violent, slightly off-center characters, also has two chart hits. His "Best Seller," featuring another ace character actor, Brian Dennehy, moved up to No. 10 while "Cop" crept up to No. 15. Oscar winner "The Last Emperor" inched up a notch to No. 7.