If you liked
Toaru Majutsu no Index
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...then you might like
Durarara!!
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Both stories have very big settings. Both stories feature organizations' conflicts Both stories have some creepy characters. Both stories have massive amount of characters. Currently, Index>Durarara!! though.
If you liked
Bakemonogatari
|
...then you might like
Denpa Onna to Seishun Otoko
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Similar mood and setting. And, well, instead of Youkai, we have "Denpa". Being made by Shaft, both drawings are very similar to each other. Protagonist has harem. Somehow the whole story just seems to be an alternative version or inspiration to bakemonogatari. "both Denpa Onna and Bakemonogatari are intelligent harems for adults with good taste." stated by McRib
If you liked
Suzumiya Haruhi Series
|
...then you might like
D-Frag!
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In both series, the protagonist is forced to join an extraordinary club. Protogonist in both stories is a Tsukkomi type, which helps to create humor in the story.
If you liked
Shakugan no Shana
|
...then you might like
Hidan no Aria
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They both have the "boy-meet-girl" trend. They both feature a tsudere, shorty, and powerful (in terms of battles) character voiced by Rie Kugimiya. Both produced by J.C. Staff.