These are my all time favorite anime/manga, the absolute best of the best.

First Tier Rating
Black Butler (manga) Masterpiece
One Piece (manga) Masterpiece
Eiichiro Oda is a genius, that is all I have to say. He's dropped hints and foreshadowing to events 10 years before they become important. 10 years. Unlike other long running shonen, Oda never loses track of the story. He doesn't let the story wander aimlessly, he has it mapped out, and he knows how to make it important and fun. But arguable most important, One Piece is a multilayered story. On the surface, it's an adventure story about a small group of friends exploring the world and following their dreams. But underneath that, it's about racism, slavery, and a corrupt tyrant. Because of that, One Piece is one of the few things that anyone could truly enjoy. People who are 10, 30, male, female, otaku or non-otaku could fall in love with it.
A Bride's Story (manga) Excellent
I have never seen a graphic novel with such stunning, detailed, or beautiful art. It seems as though Kaoru Mori draws every thread in the characters intricate period clothing. Her female characters manage to do something very hard: be historically accurate AND strong. Any author that manages to do that has my respect. Everything is historically accurate (Regrettable, I know very little about Central Asian history and culture, so there could be inaccuracies, but it's correct as far as I know). As a history and culture nerd, I love stories like this that make me learn in addition to entertain me. The plot is very slow and, dare I say it, boring, but I can forgive it for that because I simply enjoy spending time with these characters.
Cirque du Freak (manga) Excellent
A manga adaptation of a series by my favorite author? YES PLEASE! Darren Shan is my favorite author because he's not afraid to do unconventional things. Maybe he'll kill off the main character and let the bad guy win. Or maybe the bad guy will be defeated and our hero will become the new villain. Or maybe the world will be destroyed by celestial demons. Anything can and will happen! Because of this, I actually worry about the characters. I've never been afraid for Luffy or Lupin because they always win and there are minimal consequences in the few cases they don't. But I was legitimately scared for Darren (the MC) many many times. Having a suspenseful story is a nice change of pace once in awhile. Anything can happen also applies to the plot. Plot twists abound, and there are few series that have blind sided me as often as Cirque du Freak. The icing on the cake is that the twists actually make sense and don't come out of nowhere.
Durarara!! (manga) Excellent
There's nothing I love more in a story than a good plot twist, and Durarara!! has plenty. No one is what they seem and anyone could do anything. Add to that the fascinating and fun to watch characters and it only gets better. Add to that the sheer uniqueness of the story and it becomes amazing. The shout outs to other manga/anime, such as the 4 page long Black Butler (one of my other favorites) reference, don't hurt either.
Maoh: Juvenile Remix (manga) Excellent
A problem I have with a lot of manga is the unrealistic idea that "Love and friendship will over come all!". Maoh doesn't have that problem. The characters need to use their brains to overcome obstacles, and there are consequences if they can't. It manages to be realistic even with superpowered characters. The realism carries over into the plot as well. Political battles are waged, not typical shonen fist fights. The superpowers are unique. There's no super strength or speed. Instead, there's ventriloquism, being able to push someone, or simple luck. Those may not sound like cool powers, but they're used in intelligent and interesting ways.
Tegami Bachi: Letter Bee (manga) Excellent
Tegami Bachi manipulated emotions so well. The hopeful innocence of the main characters, the innate sadness of their world without light, the joy the people feel at things we take for granted, the subtle horrors of the character's experiences; all of these infect the reader in varying and complex ways. At the same time, it's cute yet scary, sad yet hopeful, so dark yet filled with light.
Soul Eater (manga) Very good