Comparing Amoongus to Ferrothorn.
Scarf Kyruem Blizzard versus max/max+ Amoongus: 75.46 - 89.35%
Scarf Kyurem Blizzard versus max/max+ Ferrothorn: 36.07 - 42.89%
Scarf Kyurem Hp Fire in neutral weather versus max/max+ Ammongus: 29.62 - 35.18%
Scarf Kyurem HP Fire in neutral weather versus max/max+ Ferrothorn: 56.81 - 67.04%
Hippowdon Earthquake
...versus max/max+ Amoongus - 43.51 - 51.38%
...versus max/max+ Ferrothorn - 35.79 - 42.61%
...versus max/max+ Gliscor under Gravity - 36.44 - 43.22%
...versus 240/216+ Swampert: 41.89 - 49.37%
Landorus Earth Power
...versus max/max+ Amoongus: 37.03 - 43.98%
...versus max/max+ Ferrothorn: 35.22 - 41.47%
...versus max/max+ Jellicent: 33.41 - 39.35%
...versus max/max+ Eviolite Chansey: 12.78 - 15.05% (for the lulz)
...versus 252/0 Espeon with Light Screen up: 30.53 - 35.92%
252 Naive LO Tornadus Hurricane versus max/max+ Amoongus - 94.9 - 112.5%
Amoongus may not be at the top of the pack, but what I believe sets it apart is the typing. Poison isn't bad as a defensive typing, and being neutral to Earthquake only helps.