Burris AR P.E.P.R. is a great choice in the affordable range. It is available in a quick release for under $100. I have 2 of the fixed mount versions.
In the next price bracket you have American Defense Manufacturing. They make several models with different degrees of forward offset to account for various sized scopes and different needs with regard to eye relief. If you're military or LE, I know of a site with a fantastic discount on these. I have two of these also.
Next price level above that is Larue. They also have several different models. I personally don't see what you get for the extra money, especially considering the discount mentioned above.
The top of the line, price wise, is BOBRO. Again, don't see what the big deal is and they are spendy.