Highest damage from a single auto attack (counting Yi's as two separate strikes here) ignoring the farming from Nasus and Sion (and Veigar with Lich Bane) actually belongs to Poppy.

40% damage buff to everything she does is the highest damage buff in the game.

She has 114 base AD at 18.

Items for this will be 1x Infinity Edge, 4x Bloodthirster, 1x full stack Sword of the Occult. Also red pot.

Total AD: 590+28+114=732

Full Bloodlust Trynd gets only a 36% damage boost to criticals, so Poppy will still out damage him.

Using Nasus's Spirit Fire (-40 armor reduction), Corki's full gatling gun (-50 armor reduction), full stacks of level 3/4 Heimer turrets (-50 armor reduction), and Evelyenn's Ravage (-26 armor reduction), that comes out to -166 armor. Add in Stark's Fervor and full Black Cleaver, and that's an additional 80 reduced armor, which is -246 reduced armor.

Level 1 Annie has 8.5 armor, so this brings her to -237.5 armor. Which is a 70% damage increase.

Adding Lethality mastery and Runes is another 55.51 crit damage increase.

This brings her total damage against a level 1 Annie to 7064.

I reject your reality and substitute my own!

-Adam Savage, The Mythbusters