The more I go back and rewatch episodes of The Big O, the more I can't help but think that it's one of the most mature animes ever made. Not so much in terms of the over-arching story, which is merely a packing together of various scifi tropes in a cryptic, nonsensical way, but in the characters and the stories that populate the episodes in between. Basically, I think this is a series that's strength lies more in its episodic episodes than in its story episodes. Although, that's not entirely the case.
The episode I'm watching right now is Winter Night Phantom and, even though it's focus is on a secondary character and it ultimately has little bearing on the main story, it's just so, so good. The way it looks, the story, the characters, the music, it all just comes together to achieve something close to perfection. That probably seems like a strange thing to say about a mecha anime, especially one with bombastic battles in a number of episodes, but this episode in particular could almost be mistaken for the episode of some noir series.
I've seen very few other animes that can come close to The Big O when it hits all the right notes, and it does that more than once.