Depends on many things.
-how easy/cheap it is to access spare parts and ammo for said rifle
-what country is making it.. is it your enemy lol
-where you plan on using it: water, desert, jungle ect.
-what temperatures you need it for: frozen Canadian or Afghani wasteland, or humid jungle
-what you'll be doing (patrolling cities and not wanting ricochets and bullets tearing through targets and hitting civillians) or will you be in the hills firing at long distance targets
-how easy is it to maintain
-do you teach your soldiers fire discipline (most NATO countries except US) or do you want to enforce semi auto only (US)
-will it be a club when the soldier is out of ammo, or need a bayonet
Canada, Denmark, NL and a lot of other countries use the c7, which is basically a US infantry copy except made by Colt Canada and is full auto. So I guess Colt makes the best military rifles, since almost everybody is using them must be easy access to parts and equip, but obviously only if you're an ally of the US. FFL uses FAMAS but is replacing it, not with Colts as the French don't want to support the US arms industry, as they have their own arms industry.
Here's the Pak border guy's making weapons out of scrap metal, too bad this area is wholly deserted now since the Pak taliban moved in and shut everything down:
http://youtu.be/FinRqCocwGE 
