The Big Gunnzbowski
Damas y Caballeros...
Depending on what we're basing our criteria on, I'm going to say:
The Great Mighty Poo from "Conker's Bad Fur Day". Funniest boss ever... Entertaining until the very last kernel. For those not aware of what I speak of, it's a giant turd that you must kill by flinging toilet paper into its mouth while it sings to you... It's attack? Spitting out digested corn at you... If that doesn't qualify, I quit.
On a more serious note, I'm going old school:
Mike Tyson from "Mike Tysons Punch Out"
His strategy was nothing like any of the other fighters in the game. He hit you once, you went down. I wonder how many NES controllers were broken because of this fight... I didn't beat Mike Tyson for the first time until 2004. I was playing on an emulator and decided to give him a try. Yes, it took me almost 20 years to beat Mike Tyson...but universally, It's got to be considered one of the biggest accomplishments in gaming ever...
The Great Mighty Poo from "Conker's Bad Fur Day". Funniest boss ever... Entertaining until the very last kernel. For those not aware of what I speak of, it's a giant turd that you must kill by flinging toilet paper into its mouth while it sings to you... It's attack? Spitting out digested corn at you... If that doesn't qualify, I quit.

On a more serious note, I'm going old school:
Mike Tyson from "Mike Tysons Punch Out"
His strategy was nothing like any of the other fighters in the game. He hit you once, you went down. I wonder how many NES controllers were broken because of this fight... I didn't beat Mike Tyson for the first time until 2004. I was playing on an emulator and decided to give him a try. Yes, it took me almost 20 years to beat Mike Tyson...but universally, It's got to be considered one of the biggest accomplishments in gaming ever...