For the best view in Paris, head to the Tour Montparnasse (Montparnasse Tower). This unfortunate 1970s black glass tower that dominates its Left Bank neighbors has an observation deck. Take the elevator to the 56th floor and then steps to the roof. The elevator ride to the top takes just 38 seconds (Europe's fastest).
There was such outrage after this tower was built that an ordinance was passed prohibiting further towers in the city center. The best thing about the great view is that you can’t see this tower! Info: 15th/Métro Montparnasse-Bienvenüe. Open daily Apr-Sep 9:30am-11:30pm, Oct-Mar 9:30am-10:30pm. Last ascension a half-hour before closing. Admission: €13.