Ruggedness and reliability first. The weapon must be able to be usable immediately under any conditions, extended storage without maintenance, hot, cold, dirty or dropped.
The second criteria is accuracy over all ranges of use, close combat to long range defense.
A third consideration is ammunition availability. Since most LE and military rifles use 5.56, a weapon that can use that ammunition can be used with whatever is dropped or discarded. You could also use the same presumption for 9 mm too but I am assuming we are talking rifle rounds.
For most of the last 30 years either my Norinco SKS (I got a good one that shoots 2MOA all day) or my Nylon 66 has been in trunks, behind truck seats,and with me when I’m far form home or civilization
They both work no matter what, are accurate enough, and most of all make me feel comfortable having one of them.
The N66 is hard to beat considering a crown royal bag full of ammo is a LOT of ammo.