Earth
Lavabending - 25% chance
Metalbending - 90% chance
Sandbending - 87% chance
Fire
Combustion bending - 25% chance
Lightning bending - 87% chance
Air
Spiritual Projection - 72% chance
Flight - 1% chance
Water
Bloodbending - 17% chance
Plantbending - 64% chance
Icebending - 100% chance
Healing Hands - 70% chance
Spirit Bending - 75% chance
I think the percentages should be this:
Earth
Lavabending - 5% chance
Metalbending - 90% chance
Sandbending - 85% chance
Fire
Combustion Bending - 5% chance
Lightning Generation - 76% chance
Lightning Redirection - 79% chance
Air
Spiritual Projection - 15% chance
Flight - .5% chance
Water
Bloodbending - 18% chance with the full moon.
Bloodbending - 5% chance without the full moon.
Plantbending - 58% chance
Icebending - 100% chance
Healing Hands - 80% chance
Spirit Bending - 65% chance
The reason why I put flight at a low "number" is because out of the hundreds of airbenders that are shown, only two were able to show the ability.
I added Lightning Generation and Redirection because Iroh explained that people can learn to generate lightning and redirect lightning. Lightning Generation is lower because even though in the LoK it was to power the city, it was still a rare skill to actually generate lighting. Redirection is slightly higher just because Iroh explained its not the same process and Zuko can redirect lightning but cannot make it.
Lavabending is lower because only two non-avatar benders were able to do so.
Healing Hands is raised up becaused the Northern Water Tribe teached it casually back in da days.
Plantbending is lowered just because the swamp people used it and because most waterbenders didn't think to use it.
Spiritbending is lowered because it takes spiritual knowledge to practice it.
NOTE: Spiritbending doesn't mean the person can regularly heal.
Unalaq couldn't heal regularly but can spiritbend.
Spiritual Projection is lowered extremely because only Jinora used it.