It had an energy, edginess, and emotion (and insanity) we haven't seen since. Oh, sometime they come close, or at least try, but it's just not the same. As well as some unique, interesting concepts.
Particularly during SM from my viewpoint, where it followed a lot of cliches seen in other anime series like Yokai Watch for example. I don't like slice-of-life storytelling in general and prefer seeing more traveling involving the main cast, which is why I liked OS and the other older sagas a lot more.
Not to mention way too much gen 1 representation nowadays, at least it's somewhat exceptable currently with them being based there... but still.
Too little gen 2/4/5 rep :/