Tour Montparnasse / Párizs

THE skyscraper of the city (210 m).
Observation deck on the 56th Floor (currently closed for renovation) and observation deck in the 59th Stock (currently open).
Note: the observation deck accessible is only accessible on foot.

Admission *** ***
Adults pay 14 EUR.

Opening times *** ***
daily at 9:30 h
to (depending on the season) 22.30 h / 23.00 h / 23.30 h

I personally found the view from here is much better than from the Eiffel Tower. Of course, you MUST have been on the Eiffel Tower, but from the Montparnasse Tower you have a view ON the Eiffel Tower - great!
My tip: best time to go there is twilight. Simply beautiful!