Can we allow a distinction between a battle rifle
cartridge and that of an assault rifle? While 99 44/100% of civilians will never have the opportunity to possess a true Sturmgehwer, that is, a select-fire intermediate-cartridge "carbine," we can possess some sort of semiauto rifle that chambers an intermediate cartridge.
The original intermediate to be adopted by a major power was the 7.92x33mm in the StG44, followed by the 7.62x49mm of the SKS (not a true assault rifle, but a tryout for the cartridge) and AK-47. The 5.56x45mm followed, with the Russians aping it with the 5.45x39mm. With the exception of some recent, unadopted rounds (e.g., 6x47mm SAW, 6.8 SPC, 6.5 Grendel, etc., plus the new Chinese round), that's about it.
The rifles chambering 7.62x51mm (7.62 NATO) are more appropriately termed "battle rifles," as the 7.62x51mm was intended to provide the performance of the .30-'06 (7.62x66mm) in a shorter case. Thus, the M14/M1A, FAL, HK91, don't really fall into the "assault rifle" (in quotes, 'cause we're talking semiauto here) category.
If we consider the intermediate cartridges, we have the AK family, the AR-15 family, plus some rifles that fell by the wayside (e.g., Beretta AR-70, FN FNC) and some new contenders (SIG-556 being the newest).
For me, while I have a safe full of AK variants, I'll have to go with the AR-15, because of ammo availability and versatility with the modular concept: one lower, several (many?) uppers, changeable faster than you can change your trousers. I will say that for rugged simplicity the Valmet (in .223) does have its appeal.
If we are talking battle rifles, I'll take the M14. It's got the sights and the trigger its contemporaries lack. Come to thing of it, sights & trigger are the big drawbacks of the AK family.
That's my story, and I'm sticking to it.
Regards,
Walt