We have many favorites.. so depends what you are looking for..this one is pricey but for that special occasion the best!
"Le Pre Catelan" which is in the 16th in the Bois de Boulogne forest. It used to be Napoleons hunting lodge)..besides being excellent it is beautiful. Take a taxi.

Le Pre Catelan

Bois de Boulogne - route de Suresne - Paris 75016
Tel : 01 44 14 41 14 - Fax : 01 45 24 43 25
Subway : Porte Dauphine
Closing days :
Lunch : Monday
Dinner : Monday, Sunday
pricey.. reservations a must and jacket/tie for men required
Click on this for virtual tour wink
http://tinyurl.com/e3crg

also for reservations this is a great site for Paris restaurants.
http://www.reservethebest.com