Mystery/Science-Fiction/Horror

[Dark Sephiroth]

Basically i'd like an anime that requires you to watch most/if not all episodes to understand it truly, like Serial Experiments Lain, Ergo Proxy or Karas. Something dark & mysterious, something out of the normal. I don't mind if the anime is widely popular or hasn't even been heard of =P

[Guido]

If you mean horror anime that involves psychological factors or either makes use of murder mystery drama, then I highly recommend you this following choice:

Higurashi no Naku Koro ni

In this one the viewer must pay attention to conversations, the characters, and the surroundings to find a way how to fit or assemble together the puzzle's pieces.
The catch is that when you at last have a plausible theory to work with in one arc, either is soldified or crushed in the next arc.

Answers are not literally given, but you must come with them from hints and TIPS. Playing the game will help you understand the series dynamics better.

[Matrim]

Yes, Higurashi seems the obvious choice. So far it's the only anime that can make me rewatch episodes just so I can come up with a new theory explaining the events in it.
Monster is also a great choice for dark and mysteriosu series.

[LostBlue]

Boogiepop Phantom

And again I will stress the PS1 Visual Novel DOUBLE CAST by Sony's Sugar & Rocket division. =)
Seriously, anyone who loved the suspense & mystery & lovable characters in Higurashi will love this game. It's the exact same genre although not Moe~ (moe~ wasn't as prominent of an existence yet back then). I don't know if anyone TL it for an english audience. If someone knows how to insert TL a PS1 game, I'm willing to TL this game.

Haven't seen it but I hear many people enjoying Eflen Lied as well for this genre

These ones probably aren't exactly what you are looking for, but Evangelion and Infinite Ryvius (Mugen no Ryvius) also fits in this genre. Both are more drama oriented then mystery. Mystery is more the setting not so much the main plot.

[Dark Sephiroth]

Originally Posted by LostBlue

Haven't seen it but I hear many people enjoying Eflen Lied as well, for this genre.

I've seen Elfen Lied, wonderful anime =D

Thank you for the suggestions, that Boogiepop Phantom sounds interesting! And i'll have to look out for that game, since me and my brother are both gamers (him more so then me though) x3

& thank you Guido =) i took your suggestion and am now downloading Higurashi no Naku Koro ni, i hope it's as good as you claim it to be. Monster looks interesting as well, was reading up about it yesterday, so i think i'll have to put that on my list as well

[LostBlue]

DOUBLE CAST is awesome story, but if you're excepting high game play you will be disappointed as Visual Novels never have high level of game play. DOUBLE CAST is the first of several YaruDora (do it yourself Drama) series that Sugar & Rockets produced. All these games have several endings (DOUBLE CAST alone has 27 bad, 3 normal, and 4 good endings) and the more you play the more choices and branches become available. One of the YaruDoras became very famous and animated, was in their PS2 series, BLOOD (yes, that same vampire story).

CM for Double Cast
http://www.youtube.com/watch?v=kBPJ8...=double%20cast

OP for Double Cast
http://www.youtube.com/watch?v=BdK7L...=double%20cast

Character OP for Double Cast
http://www.youtube.com/watch?v=HTW1P...=double%20cast

I highly recommend Higurashi as well. HIGHLY. I didn't post it in above post as it was mentioned already. Higurashi is also based off a Sound Novel (game). If you really love the mystery/suspense genre go with this.

[Paper Lilies]

Haibane Renmei. It starts out light enough, but it gets extremely dark.

[LCeh]

Le Portrait de Petite Cossette. A 3 part OVA, fits your bill perfectly. A must for those looking for these types of anime.

[Cyz]

If you want Mystery/Horror, then watch Higurashi no Naku Koro Ni. It has tons of horror and mystery. If you prefer Science Fiction, then watch The Melancholy of Suzumiya Haruhi.

[miaka]

Ayatsuri Sakon - adventure, mystery, horror
Spiral - detective case with scientific explanation but its licensed
Monster -

watch out for Death Note which will be out this coming October.

[physics223]

As a genre, this doesn't have much science fiction it it, but as a reality in the anime - it has tons: Kousetsu Hyaku Monogatari is my recommendation. It's an eclectic mix of mystery and well-placed horror, IMO.

[Dark Sephiroth]

I have enough anime to last me ages now, yay x3 (although i don't mind more suggestions =D). I will definately be checking out everything everyone has suggested (although a few at a time lol).

Thank you

[RuFi]

Paranoia Agent, Boogiepop Phantom and... Monster

Watch them all!