Welll, it's hard to top the reigning champion, black-sesame soft serve, but give kuzukuri noodles on ice with sweet dipping sauce (kuzukuri no kuromitsu) a try. Kuzukuri is made from arrowroot starch. Cut into wide, flat noodles for this dish, it's crystal clear and totally flavorless... but then you dip those noodles in "dark honey" or brown sugar dipping sauce, and slurp them up like they're sweet, ice-cold udon. Mmmm.
In some areas, a variation of this dish is made with tokoroten, a seaweed-based version of gelatin made from agar.
Mad props to Georg K. for suggesting I try these. Totally unlike anything I've ever eaten, and a completely addictive summer treat. To think I was going to go for the cheesecake! Time to get a jelly-noodle press. Here's a how-to for the tokoroten version, which yields thinner noodles: