What do you guys think is the most buffed ability in history?

I'd probably have to go with Nasus' Ultimate. From what I gather, on release it started out dealing percent max hp to surrounding enemies, and Nasus gains a portion of the damage he deals as bonus attack damage for 15 seconds, at the cost of 150 mana.

That's it. No bonus health, no bonus resistances, just that.

On Patch 0.9.25.34 it was buffed to give 300/450/600 max hp on cast. The percent health AP ratio was lowered from 5% per 100 ap to 1% per 100 ap.

On Patch 3.02 his ultimate now grants him 50 bonus attack range, and 100 bonus cast range (cast range later reverted in 4.1), and its mana cost was lowered to 100.

On Patch 6.21 his ultimate now grants him 15/35/55 armor and magic resist on cast, with an additional 1/2/3 each second it's up. This is a total of 30/65/100 by the end of the duration. He no longer gains bonus AD from dealing damage.

On Patch 7.15 his ultimate now cuts the cooldown of his Q in half, essentially doubling his damage output for 15 seconds.

Sure, it probably hasn't received the largest number of buffs throughout the game, but the buffs it has received are truly spectacular in comparison to how his ultimate was released.