Stealth Rock. No other move in the game is capable of instantly relegating certain Pokemon to lower tiers.
I mean, if there was no Stealth Rock, Pokemon like Articuno would be in OU. The very tiers of competitve Pokemon have been influenced by Stealth Rock.
Articuno wouldnt be ou even without stealth rock ice isnt a good defensive typing since it lacks good resists and is weak to common things like fire fighting, rock and steel etc.
While all of these options seem appealing, in my opinion: Pursuit is the most powerful move in the game, mainly because of its ability to hit nearly any switching pokemon. The pokemon that benefit most from Pursuit are those with either Technician or a Dark Type. The Dark Types have an advantage in using pursuit because they will take resisted damage from Ghost and Psychic and deal supereffective damage to them. Pursuit becomes a 120 Base Power move if the opponent does not switch; if they do switch, Pursuit's power doubles to a whopping base 240!
40 * 1.5 (STAB) * 2 (super effective) = 120.
Double that for switching out.
Articuno wouldnt be ou even without stealth rock ice isnt a good defensive typing since it lacks good resists and is weak to common things like fire fighting, rock and steel etc.