This is my favourite mainline Touhou game, although not by a large margin; there are plenty of good games in the series, after all.
It's all about the impression it left me.

To have a bit of context: this was the latest game released before I had to wait for future ones like most of the fans who catched up. And by god, it was quite climatic.
-A pointdevice mode that forces you to not die at all by replaying the very section you died on.
-Absolute bangers being played across the mayority of stages and bosses (or at least they felt strong)
-I didn't finish the story with all characters nor I remember it, but the audacy of actually going to the moon to stop an invasion makes it feel like a high-stakes game for once
-Clownpiece, Junko and the whole moon setting alone carry the battles. Clownpiece for the infinite madness and Junko for being tired of your bullshit and having the purest, simplest but scariest danmaku (plus her theme).
-The euphoria of, at last, having finished all the mainline Touhou games.

To put it simply, I had fun.