This is among my favorite anime, I think it was one of my first exposures to an expanded universe.
The Concept of several anime series all existing in the same universe and tied into one another was fascinating to me.
Accelerator was one of my favs, probably because I liked how he was presented to be so powerful yet he seemed to change as the story progressed. In a way he kinda reminds me of Hyoubu Kyousuke from Zettai Karen Children. The way he is presented as a powerful antagonist but has a lot of potential as a protag.
Unfortunately I fell behind on the series, Didn't see much beyond index II and railgun II.
I would need a roadmap to figure out what order to rewatch everything again....
My favorite season in the franchise (anime-wise) is Railgun T and while some parts of it happen at the same time as Index Season 3 you don't need to watch that first... Actually there will be a big surprise if you watch Railgun T before Index 3 because of the retrospective foreshadowing.
Haven't seen it yet, the only thing I know about what I haven't seen yet is there is a girl with a name that is similar to railgun who has a remote control that she bludgeons people to death with..... I think....
Yeah, Shokuhou Misaki. The other Level 5 in her School... She actually appears in the first episode of Season 2, and she's not a friend of Misaka Mikoto. She plays a big role in Season 3, and even the Manga arc that follows that.
She appears in Index second novel series: "New Testament" and she has a full arc dedicated to her there. So she's quite popular, and one of my favorite characters. (Not the top though.)