This is definitively my anime-of-the-season, and although that opinion is likely not going to be unique to me, it's pretty easy to see why most think this. What began as just an insanely colorful and wacky animation eye-gasm developed into a compelling plot that really gripped me week-in and week-out with its own strange way of storytelling. The plot comes and goes, interspersed between sections of fantastical settings, animation, and transformation sequences, never quite giving you the full story and lacing it with imagery and metaphor, at least until the last few episodes. Although some are sure to say that the "tell-all" point at about episode nine and ten cheapen the experience, I find that they were necessary given just how far the show goes into symbolism. The big story reveals near the end honestly felt like a mini-Evangelion, which is something I never thought I would find in anime in this day and age.
Characters were also all pretty great, with our two mains (Cocona and Papika) getting an extreme amount of development and the rest being explored at least a fair amount with respect to the plot. This is hard to talk about without spoiling anything, which would certainly ruin your experience, so I'll refrain from getting into the details, but pretty much everything gets tied up by the end so don't fear if you "don't get" something as you begin to watch it. There's also some heavy undertones of lesbian sexual self-discovery, and although nothing explicitly described (this is Flip Flappers we're talking about), it's there. Our main characters coming to terms with these feelings is a core part of their characterization and the story at large, so just be aware of that as you watch. It's nothing that truly devalues the experience if you choose to ignore it and not read into it.
I talked up the aesthetics super hard in my season preview, but I'll reiterate: the entire show is absolutely fucking drop-dead gorgeous for a TV anime. There are some wonky cuts near the end, but they only happen during instances of incredibly heavy and crazy movement. They're likely to get cleaned up in the BDs, SHAFT-style, so I'll forgive them. Studio 3Hz did an amazing job over this series' 13 episode run. Music was also to die for, incredibly fitting and atmospheric, completely bringing in the hype when it needed to as well as having probably the best OP/ED combination of the entire season.
Flip Flappers gives me newfound hope for new anime-original IPs, especially in a season where Izetta and Matoi flopped so incredibly hard. Easily my AOTS.