Oh, thats a rather good and new question. At first sight, I’d also go for the “House of blue Leaves”, but not mainly because of the fight against the Crazy 88, but because of the “Lady Snowblood” inspired duel at the end. This fight in the snowy japanese garden would make an awesome showdown to almost every movie. Just stretching it a little more can’t hurt, I really liked the meditative style it was shot in.
But also the Chapter of “The lonely Grave of Paula Schultz” of course combined with “The cruel Tutelage of Pai Mei”-Chapter would make an awesome thriller with asian influence. The scene, where the Bride frees herself out of the grave is classic. Then this music (“L’Arena”) by Ennio Morricone got my skin crawling with a sense of utter, glorious victory. Would also make a great scene for a Showdown.