Since this is the Full Auto forum, I presume the term "assault rifle" applies to selective fire weapons, in which case I would have to say that the M16 is the best of the bunch today. The AK is good, but the stock is poorly shaped for control in FA, and the FAL is a full power rifle which also presents control problems in FA fire.
If the definition is expanded to include SA only "assault rifles", then both the SA FAL and the SA M14 clones are good.
Jim