The answer would be Picatinny.

I've played around with all of the available combinations out there and found the GG&G Aimpoint mount to work the best for proper dot location and my taste for where the front sight post sits in the FOV window.

Personaly I like the dot on top of the front sight post, and I like the dot to be almost perfectly centered in the FOV window. The Aimpoint QD mount puts the dot in the upper 1/3 of the FOV window. If you use the M4 spacer it changes that to the lower 1/3 of the FOV window. That would be my preference if I had to settle for something less. If you use the newest Aimpoint QD mount then your stuck with the lower 1/3 FOV location because theres no longer a spacer required.

The problem with using the GG&G ring is you must mill a cross bolt slot in the forward carry handle base if you want to use the M68 flip up covers as designed. The added benefit is you get a spare battery compartment right on the 30 mm ring.

The ARMS QD ring works well too but still puts the dot relative to where the Aimpoint ring does.