Toaru Majutsu no Index Movie: Endymion no Kiseki

if you're confused on how to watch Toaru franchise, here's probably the best order that I can think of

This series is recommended to be watched in release order.

My interpretation of the Toaru Series Watch Order.

A comprehensive watch order of the Toaru Series series, which includes every entry of the Toaru Series franchise.
Original Guide: https://myanimelist.net/forum/?topicid=1867550

Release Order

Follow the items list order.

Chronological Order

According to the Index wiki, the chronological order for the series is as followed:
(version with links in the original guide thread comments)

Level Upper Arc
Railgun episodes 1-14.
Railgun OVA episode 1.

Index Arc
Index episodes 1-6.

Big Spider Arc
Railgun episodes 15-16.

Deep Blood Arc
Index episodes 7-9.

Poltergeist Arc
Railgun episodes 17-24.

Sisters Arc
Railgun S episodes 1-9.
Index episodes 10-14.
Railgun S episodes 10-16.

Silent Party Arc and Angel Fall Arc
Railgun S episodes 17-20.
Index episodes 15-17.
Railgun S episodes 21-24.

Three Stories Arc
Index episode 18.
Index II episode 1.
Index episodes 19-20.

Kazakiri Hyouka Arc
Index episodes 21-24.

Necromancer Arc
Accelerator episodes 1-12.

Orsola Aquinas Rescue Arc
Index II episodes 2-5.

Toaru Majutsu to Kagaku no Ensemble
Visual Novel for the PSP (optional).

Tree Diagram Remnant Arc
Index II episodes 6-7.

Toaru Majutsu no Index Movie
Index Movie: Endymion no Kiseki

Daihasei Festival Arc
Railgun T episodes 1-3.
Index II episodes 8-13.
Railgun T episodes 4-15.

La Regina del Mare Adriatico Arc
Index II episodes 14-16.

Dream Ranker Arc (Introduction)
Railgun T episodes 16-17, 19.

Academy City Invasion Arc
Index II episodes 17-23.

Skill-Out Uprising
Index II episodes 23-24.

Dream Ranker Arc (Indian Poker)
Railgun T episodes 17-20.

Document of Constantine Arc
Index III episodes 1-3.

Battle Royale Arc
Index III episodes 4-6.

Dream Ranker Arc (Doppelganger)
Railgun T episodes 21-25.

Acqua of the Back Arc
Index III episodes 7-9.

British Royal Family Arc
Index III episodes 10-14.

DRAGON Arc
Index III episodes 15-17.

World War III Arc
Index III episodes 18-25.

Note: The Dream Ranker Arc is split into 2 entries because it covers multiple time periods. You can ignore Dream Ranker Arc (Introduction) and view Dream Ranker Arc (Indian Poker) as episodes 16-20 if you'd like.

AEGC Interest Stack index: https://myanimelist.net/forum/?topicid=2017008

The world of Index is a fascinating one where the main conflict is between science & development and magic & religion. The main storyline is covered within the 3 seasons of Index. It's perfectly fine to just stick with that, but you can also add in several side stories and spinoffs (that are arguably better) to really expand the world and it's lore. But that does often bring up the question, which order to watch them all in? Do you watch all of index first before touching the spinoffs, or do you go for Railgun instead as it is widely considered the superior series? And what about Accelerator?
Don't worry, I got you.

I recommend the (simplified) chronological order over the release order

(watching by the actual chronology means switching series every few episodes... sorry, I'm too lazy for that)

A compilation of anime that began airing during the month of February.

This Month is most notable for airing the original Dragon Ball, Naruto: Shippuuden and the directorial debut of Satoshi Kon with Perfect Blue (premiered in August, but MAL logs it's theatrical February release so I'm keeping it here lol).

See Also:

January: https://myanimelist.net/stacks/15005
March: https://myanimelist.net/stacks/15017
April: https://myanimelist.net/stacks/15029
May: https://myanimelist.net/stacks/15033
June: https://myanimelist.net/stacks/15041
July: https://myanimelist.net/stacks/15044
August: ttps://myanimelist.net/stacks/15051
September: https://myanimelist.net/stacks/15053
October: https://myanimelist.net/stacks/15101
November: https://myanimelist.net/stacks/15102
December: https://myanimelist.net/stacks/15104

Release order. Excluding OVAs + specials at the moment.

The Broadcast Order to watch the A Certain Magical Index, A Certain Scientific Railgun & A Certain Scientific Accelerator