Rock solid - Aero Precision Schmid Ambidextrous Safety Selector

This is best for absolute simplicity. Overall it's a modified GI safety selector, they just cut the one end to accept the screw. Unlike others, it uses a socket-head screw which is far less likely to round off than the usual counter-sunk ones (I was able to get a decent amount of torque going when I tightened mine). There was virtually no slop between any of the pieces either. I also prefer 90-degree selectors since there's basically zero chance of it being bumped off of safe and it's nice to not pay for features I don't plan on using.