"Toaru Kagaku no Railgun S", the second season of this spinoff, is truly a huge success. It primarily covers an arc that was briefly touched upon in the first season of the main series "Toaru Majutsu no Index", but with much more detail and interest. I'm very glad I watched this season before the main series ! it's very well-structured and has excellent development of the main storyline. The plot, which is initially simple and easy to follow, becomes intense and gripping throughout the episodes, managing to touch me emotionally and keep me hooked.
The fights are extremely relevant and exciting to watch : they're not just there for action's sake like in some other shonen anime. This arc is truly superior to those of the first season, it represents the peak of the entire franchise for me. Go for it, you won't regret it. I highly recommend watching this spinoff, or at least watching this second season before diving into the original series, as it really makes the characters' actions and feelings much more logical. But if you've already watched or read this arc in Index, it's not a big deal, as it adds a lot more depth to the story.