You may want to take a look at Hotel Relais St. Germain. It's more of a four-star boutique than a bonafide five-star property, but I really enjoy it, and have a couple friends who absolutely swear by it. Also, if you're vacationing with a wife/significant other then I think it really hits the whole 'romance and relaxation in Paris' theme well. Great restaurant downstairs, lovely small lobby that feels like a clubby living room. It's also very nicely priced (I've stayed in their terrace suite, which, though not really a suite, was around 400euro and decorated in contemporary style, but not overwhelmingly so). Location is one of the better ones in the city.

Don't book anything smaller than a junior suite though.