The only time we actually see an AT-AT in battle in its final form is at the Battle of Hoth. There, the only available firepower comes from turrets and snowspeeders, all of which are too weak to penetrate the walkers' armour. Anything stronger is deemed too valuable and is either being evacuated or forced to be abandoned as they're not defrosted from the earlier blizzard yet.
Excluding prototypes and Force-related stuff, just how strong is that armour? What armaments have proven capable of penetrating AT-AT armour and inflicting significant damage on its own without relying on external support to soften or disable the armour first?