とある科学の超電磁砲T レールガン603発目

風の谷の名無しさん＠実況は実況板で

Redditで見たとあるシリーズお勧め視聴順、理由も含めてほぼ同意だわ
My own recommendation is to go **Railgun > Railgun S > (Railgun T) > Index I > (Index Movie/Accelerator) > Index II > Index III**.
You might miss some slight referrals to the parent series by starting Railgun first,
but the somewhat problematic adaption of Index means that you will get much more interest in the series by following the "Scientific Side" of Academy City
in Railgun first before heading into　the"Magical Side" in Index, despite Railgun being the spin-off.
This is exactly what I did back in 2018 and I wholeheartedly recommend this.
Alternatively you may opt to watch the first 6 episodes of Index I
to get a brief introduction of Academy City
and the series' MC before heading into Railgun. Due to one shared arc between the two afterwards,
don't go beyond there until after you watch Railgun S.
For Railgun T the anime production team does try to cater for those watchers
who only watched the Railgun anime (of which there are A LOT, my own sister included).
You should be more than fine with jumping onboard Railgun T straight after the two previous seasons of Railgun,
but I do recommend poking into Index during or after finishing with Railgun,
as the events in Railgun start to become interwoven with the "Magical Side" from the current season onwards.
You may also consider just reading the Index novel while going through Railgun
- the novels have all the flesh and bones of that side of the To Aru world,
and goes away with most of Index anime's faults.
The Index Movie and Accelerator may be watched after Index I.