Aux Armes De Bruxelles is a long-established restaurant near the Grand Place with an excellent menu and good beer and wine list. The moules are to die for. I ate there every evening on my last trip.
Address: 13, Rue des Bouchers - 1000 Bruxelles; Phone: +32 2 511.55.50; Website: www.armebrux.be/
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And the moules ... I'd have to say that they're only so-so.
It's probably the best restaurant on Rue des Bouchers (the others I was with didn't order moules and were generally happy with the food), but in my opinion you're better off anywhere else in the city. (That said, while I do avoid other restaurants on that street, if I'm travelling alone, I will go to Leon across the street for moules: it's a good place to dine solo and it's conveniently located near Delirium Cafe/Floris Bar.)