I haven't tried too many around the city, but last fall I had some tasty ones at Jane. The mussels were almost too salty, but the broth was very flavorful aside from that, and the slight hint of rosemary on the fries was really, really, great, especially dipped in the mussels liquid.
I'll second Bruxelles, and add Markt and L'Enfants Terribles and Casanis. I've never had either an amazingly good or an amazingly bad version of moules frites, come to think of it. They've all been fine. Of course, I'd probably eat a stick if it came in a saffron cream sauce.