The Big O, because of the characters who are very unique and all essential to the plot and story and developing it in some way. The story is phenominal with mysterious atmosphere. The sound track is pleasing to the ears using classical music and amazing trumpets. Lastly the art style is very unique, taking an art style from Batman: The Animated Series and combining it with anime creates a very simple but also complex artstyle. The mechs are very nice and as well looking similar to the classic designed mechs such as Tetsujin 28. The style is very gothic but without being to edgy or angsty with some noir thrown in. The only complaint is that the ending is very confusing. If it wasn't for the ending the series would be perfect.
Death Note and Mirai Nikki because I love how we, as audience have to be paying really close attention to the anime in order to understand what's going on. I love how the author is able to make the characters do such unexpected things that well, if it was me, I wouldnt have been able to think of such things! The animes are so mysterious, the way the characters "Think" is amazing! I really like those two animes!