Best is hard to qualify. Some rifles excell in accuracy or lethality or reliability. A rare few excell at all the qualities one looks for in a weapon of warfare (reliability, durability, lethality, accuracy - in order of greatest importance). Others excell on more dubious rounds (The look like they shoot lasers is a favorite quality of mine).
But, were I forced to choose, I would select the one that has had the greatest impact on the world, and in such a case one can halt their search at the venerable AK-47. Used by more nations than any other weapon ever built, this rifle has been used in virtually every conflict in the world since it's introduction on the world market. While upstaged in terms of lethality or accuracy by any number of weapons, it is the reliability of the weapon and it's capacity to absorb mistreatment that makes it such a favored weapon around the world. There isn't anything sexy or high-tech to be found - just pure, simple mechanical perfection. The fact that the weapon is produced by so many different arms manufacturers and incredible ease of construction means the weapon can be had for a relative pittance, often a tiny fraction of the price of a more advanced or capable rifle.
Of course, were I to choose the weapon that best exlempifies the idea of an assault rifle, I would undoubtedly choose the FN-SCAR. The rifle is prohibitively expensive thus ensuring it will never reach widespread proliferation, but it represents one of the handful of modular weapons on the market. Able to fire either the 5.56x45mm NATO (one of the most common modern assault rifle rounds) or the significantly more powerful 7.62x51mm NATO (similar to a .308 winchester - a common hunting rifle round. Also, a common round in sniper rifles and medium machine guns) the rifle can be equipped for a variety of roles. Tactical flexibility is only part of the package, it is also supremely reliable, showing fewer stoppages in extreme use or hostile environments than most other rifles. Accuracy and range are no slouch either, with effective ranges from as short as ~300m (carbine varient, 5.56mmx45 NATO) to greater than 700m (sharpshooter varient). With less than 1/2 MOA mechanical variation in accuracy (translating to a mechanical accuracy of about 1/2 inch deviation at 100m. The M-16, only reknowned for it's accuracy only manages 1 MOA). In a single package you have no compromises in performance, so long as you're willing to pay the hefty price for the rifle.
