Depends what you mean with Best.
If its the hardest I would like to postulate: Cruel Lajos - MOONSCARS: Jesus, It really gave me a headache. In all my years as a gamer (im 38) I never was so close to throw a controller against the wall.
If its the coolest, then: The Boss - MGS3: the way it plays, the music, the difficulty, the significance. Everything in this fight is on point.
"We all make choices, but in the end, our choices make us" - Andrew Ryan, Bioshock.