If you only have one gun forever...

[Wallace's]

AR15

 

[Max_Gain]

My Sig Gen 2 556R would probably be my choice.
It takes AK mags and ammo, which is everywhere. It is light, accurate, and robust.
It is not finicky with regard to ammo or lube, and I am pretty sure you can run it dry for a long time before it will fail.
Due to its piston design, it stays pretty clean as well.
A cheap red dot allows for a full co-witness which is also very nice.
The stock folds in so it becomes pretty small and easy to stow.

 

[AndroGuns]

FTW! G19

 

[Max_Gain]

FTW! G19View attachment 804611

That is TWO guns!

 

[Swann25]

sig mpx 9