Re: which micro red dot?
Depends on what it s going on and if you need NV capable.
AR - do you like lower 1/3 or absolute co-witness? Either LT or ADM.
MP5 - use the B&T rail, ADM mount, or the Aimpoint fixed T1. Regardless, go as low as you can.
On a magnified optic at the 1 o'clock? LT 787
Good luck