Clearly not many people know much about guns here. As with say, a longboard, there is no such thing as the "best" assault rifle. Picking an assault rifle is entirely dependent on the current situation and personal taste. Me and my friend have discussions about this fairly often, he prefers the M4, and similar weapons, which allow for lighter rounds, hence the ability to carry more with you, as well as general advantages of a 5.56mm round. I on the other hand prefer battle rifle style weapons such as the M14. I don't feel confident shooting at the enemy with a glorified .22. In my opinion, the best era of war was WWII, where they cared less about the little features of weapons, which really don't matter, and more on knocking for the enemy with one shot. I also appreciate the ability to reach out further than most 5.56mm weapons can and still inflict some damage.
With that said, as mentioned above, I really don't like assault rifles, since they compromise range and power for rate of fire and ammo capacity. So I'd say an M14 with a EBR stock. I really wish everyone would shut up about the Tar/Tavor, since it is pretty much the Halo of assault rifles, it has brought nothing new to the table, but everyone still seems to love it for some unknown reason. Bullpups are supremely overrated. The SA80 blew so bad, that the brits needed HK to fix the entire design so it didn't jam more than a 'nam M16. If I have to choose and assault rifle, however, it's going to be some variation of a G36 in 6.5mm Grendel.