Supposedly during WW II the Germans had a very healthy, and almost mystical, fear of the M2 and its capabilities.
Allied artillery holds that crown.
As far as the .50 BMG capabilities. It all depends on the type of ammo being used.
Being a large enough round it comes in all kinds of flavors. From regular Ball, to armor piercing and HE and Incendiary.
For instance, a regular Ball .50BMG will not explode you, but take the same cartridge and fire a HE bullet and it will most definately explode it's target. Read the wiki entry and go to the "Military Cartridge" types and it will list all kinds of .50 BMG flavors.