SPOILER ALERT!!!!!
Episode 96 was good because you know, all the events. But I liked 97 as well because Elza finally admits to others that she'd just been using them. It was sweet.
Episode 98 because Yuzu & Lily (I ship them and it was a really fangirly episode)
Episode 99 because, sure, the SubaYume moment, but also because Yume finally defeated Hime. I was almost crying.
Episode 36 was dramatic, but I'm a bit disappointed Yume got rid of her powers. I wish she learned to control them. Not exactly my favorite.
My favorites are either episode 96 or 1. Episode 1 because when I was sad the first seasons of Aikatsu ended, I (almost) completely forgot about when I started watching episode 1 of AiStars. There was the first encounter of SubaYume, saw Yume's powers, and just the whole episode in total. Oh and also 99. It was amazing.
So um, if you don't want to read all that, it's episodes 96, 1, and 99.