Maybe on the Iena Bridge between the Trocadero and the Eiffel Tower? It's a bridge but also has two beautiful landmarks on either side. Maybe the Jardin du Luxembourg?

Or maybe in a quiet part or an uncrowded fountain in the Versailles gardens? The staff at Versailles may cooperate with you by shutting part of the parks down just for you so you could make it really special, if only because they will find it such a romantic and wonderful gesture.

My uncle propossed to his wife by closing a rest area on a quiet stretch of motorway, with the permission/cooperation of the authorities who seemed very happy to assist. With a few friends, they spent the night painting an enormous, "[name], MARRY ME!" and took up his future-bride in a jet to see the message with ring ready.

For all the lovely ladies here, do you prefer something extravagent/amazing (i.e. 500 Parisians spelling "Marry Me" at the base of the Eiffel Tower with you at the top) or something simple (i.e. quietly on Pont Alexandre) when your future-husband proposes?