I support I Am a Hero for this category. I tend to have a soft spot for loser-y guys and characters, so I can't help but root for the main character of this manga. Couple that with a zombie apocalypse, paranoia, hallucinations, and other psychological elements and what you have is pure win.
If you are talking about the anime then it doesn't fit here, this is the Manga Award. The sound Novel wouldn't count either. If you are talking about the manga then you need to specify the title, as there are a lot of Higurashi mangas out there like f.ex. Higurashi no Naku Koro ni Jan.
Oh well...in that case I think I'll nominate Higurashi no Naku Koro Ni: Watanagashi Hen. Since there's so many chapters I can't decide which one's the best, but I like the mystery in this one the most so there XD
Currently watching: Level E, Densetsu no Yuusha no Densetsu
Currently reading: Gantz, Claymore
I will go for Shingeki no Kyojin. It's new yet already captivating. It has both the horror element with the giants eating humans and the mystery since we have no clue where those giants came from nor what they want.