Steyr AUG A3 M1 Tactical Assault Rifle
CZ BREN 2 assault rifle
Heckler & Koch HK MR Assault Rifles
PALMETTO STATE ARMORY AK-103
IWI CARMEL Assault Rifle
IWI Galil ACE Assault Rifle
Accuracy International AS50
Heckler & Koch G36
FN SCAR
Armalite M16

A rifle's power is actually determined by the cartridge it fires. Since most of these guns use the 5.56x45mm NATO, they're not more "powerful" than other AR's. Better, maybe. More reliable, more popular, more versatile, etc. But not more powerful.

Best assault rifle (and some other guns) must be cheap , accuracy , fast production , do high damage to the target , easy to carry around , good size , can add lots of attachments , light ( not heavy) magazines , high fire rate , can work on anywhere in the word ( like forest , snowy season or muddy place etc) , can share the guns with all the soldiers (fast), good suppression , easy to understand how gun works , easy to fix , Good recoil ( low ) , highly recommended , good iron sight , bullet can reaches the target that is far away from the shooter , and no sound when reloading. That is it I guess