Need some help from the experts: what SINGLE move would you add to any non-OU/Uber pokemon that would make them go up a tier or two?

There are a lot of pokemon who could perform better if given a specific move that compliments their stats and abilities. Which ones do you think would have the most impact on the current metagame?

Something like Recover on Swampert would help tank, removing the need for Rest. Or Head Smash Aerodactyl, to provide some much needed power. Any calculations that you can provide would be helpful.