Improved Matech BUIS...

[M.45]

With my AR15s, I've found the Matech BUIS is pretty solid with the exception of the mounting bracket it comes with. The screw can be over torqued and snapped. These guys make a very inexpensive solid replacement mount with a fairly indestructible grade 8 screw. For 20 bucks it's hard to beat.

http://shop.battlearmsdevelopment.com/

Pic of the bracket...