First, there are 52 Pokemon that are dragon type in at least one of their forms. (Bulbapedia says 50 but excludes Arceus and Silvally)
Head smash has 150 base power and is super effective against some dragon Pokemon, so it averages 166 base power.
Dragon claw has 80 base power, so outrage and +1 dragon claw both have 120 base power. Against all dragon Pokemon, they both average 233 base power.
Ice fang has 65 base power and gets boosted by strong jaw but not STAB. Only 14 of the 52 dragon Pokemon are 4 times weak to it, so ice fang averages 156 base power.
So the attack that's both STAB and super effective is strongest. What else were you expecting?
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