Something that will get you through anything and always be there to fire. Something you can always count on, no matter what. Cold, heat, wet, dirt, mud, drops, falls, bites, claws.
Is there anything in your mind, that you trust to take all that and keep on tickin’?
The fewer moving parts, and fewer automatically cycling parts, the more reliable.
I’d reckon that mechanically a fixed pin, single loaded slamfire shotgun is going to be nearly 100% reliable. The only malfunctions being user error or bad ammunition.
Of things in the realm of reason, a bolt action rifle chambered in a rimless cartridge will be very reliable.
That said most big name, widespread firearms are can be very reliable. Each design has certain specifical makes/models that will be better, and points more likely to fail and certain types of jams, but on mature designs they can usually be easily cleared.
Would you be willing to take a slamfire shotgun over something magazine fed? Is the reliablity difference that big of a factor to sacrifice so much else? Most people say usually not.