Creepiest Mugen Intro You Have Ever Seen.

[Superkingkong65]

For me it would have to be Bloody Motaro.  

Sure the background looks alright, but when Motaro stares at you, it feels like he is taking your soul.

[Mʀ. Sтᴇαl-Yoᴜя-Wαεfυ]

Creepiest Intro = Play Gurospooo

Best fucking intro ever? = http://mugenfreeforall.com/index.php?/topic/25506-tyrone-by-sookie/

[IsabellaRegaldo]

Giygas by The_None. Although to be fair his Intro itself isn't too creepy, but it is really creepy whenever you lose (or win) and see nothing but white text in all caps saying "LOOK BEHIND YOU." or some other random unnerving phrase. Overall the whole character is pretty creepy especially when played on the right stage.

[TotalDramaXtremist]

Lucy Diclonius by Crucifix. Enough said.