It's kind of a hard choice for me. I say 353, 357, and 358 were all pretty good, but I remember doing fun missions on other days and I don't remember which ones they were. Like the Infernal Engine fight, the fight with Xigbar in Olympus, and the fight with the three Windstorms.
Anyways, day 353 was fun because of the seven bosses and the scene with you-know-who. I like day 357 because that had the scene with Roxas and Xion, and though it was sad, I liked it. Day 358 was fun with the dual wielding and the scenes with Riku were interesting.