The FAL is, without a doubt, the most legendary weapon of the Cold War. Its combat record has included use in some of the most demanding environments in the world, from Vietnam with the Australians to South Africa and Rhodesia to Brazil and, of course, both sides of the Falklands War. The only platform that comes close to in reputation is the M16, which was what finally replaced the FAL in many countries and has essentially supplanted the FAL as the new "right arm of the free world."
For both rifles, the length of service speaks to the quality of the platforms, in terms of reliability, cost, and usability. The AR-10 was a good gun, but it simply came too late to the battle rifle category to compete with assault rifles. The M14 was simply old when it was introduced and was far too heavy for the era. The AUG is another good gun but it simply does not have the Cold War-era combat record to prove it. The G3 came close to the FAL and was actually far more widely produced, but was not able to achieve the FAL's legendary status; I would personally point to the G3's much more complex operating system compared to the FAL or even the M14. The original AK-47s were fairly good rifles, though simply not on the level of the FAL or M16; the AKMs were basically throwaway guns that could be paralyzed by mud. The AK-74 made a lot of improvements over the AKM, but the 5.45mm round was weak and it never saw much use outside the Soviet Union and the successor states.