I'm not an expert nor fan of restaurants - I prefer home made food.
But you can look at http://forums.icelandweatherreport.com/viewforum.php?f=12
Also you'll find at your hotel a free restaurant guide and cause Reykjavik is not that big - you might end with a choice of three-four places and just go there. if you don't like the place - like appearance or something - go to another. They all within 10 minutes walk :-)
And you didn't tell what you prefer - fish, meat, vine, music, "modern" or family style place etc.
PS. My favorite one is in Hofn. But that;'s a long drive :-)