Highly recommend the Hotel Luxembourg Parc hotel in the 6th, as you might guess right next to the Luxembourg Gardens. It's a midsize hotel I would say, five floors with about 4 rooms per floor. A small but quiet and swift elevator overlooks a central courtyard. Our room was a large L shape, with a roomy bathroom with shower that had downforce and heat (still not typical). The room had its own thermostat that worked well (my wife enjoys sauna-like temperatures).
TV had a full complement of international cable channels, not just French but also British, German and Italian - this is very important nowadays to keep ahead of breaking travel news. Room was very quiet and king size bed very comfortable (getting a good sleep that first night is really, really important). Good free wifi. Very nice exercise room in the basement, not that you'll need it with all the walking you will do. Good breakfast selection with pleasant service, in fact everyone at the hotel couldn't have been friendlier or more helpful.
I guess I need a negative. Hmmm, OK here's one, there is no chest or bureau so unfortunately you have to work from your luggage on the floor. Unless you unload onto the shelves that are built into the closet.
Very convenient location, if you're taking the RER from CDG you have two stations to choose from. Easy walk over to St. Germain du Pres area, also easy walk to the Rodin and Orsay museums and down to the Seine. Good assortment of restaurants and little convenience stores like Carrefour and Monop for snacks, drinks, even takeway sandwiches and meals.
They appear to be part of a mini-group of several hotels, so the others may be good as well.