This hotel is probably too much but gorgeous. On a square with a fountainwhich is lit at night and close to Parc Luxembourg which is beautiful to sit and people watch. On the corner of the square is a really good patisserie. Next to St Germain, so a lovely area to wander through. More expensive square facing rooms have a view of the Eiffel Tower from the balcony which is lit at night. I think it would be perfect for a proposal. Lovely part of Paris.
I stayed at the Mercure Montmarte - it is in a slightly sleazy part of town (sex shops close by) the area is akin to Soho but it was right round the corner from the Moulin Rouge, walking distance from the eurostar, close to the metro and restaurants and the rooms were very modern and huge. You could have slept 4 people in our bed!