Fate/Zero

Fate/Zero is a Japanese action, adventure, dark contemporary fantasy, drama, tragedy, battle royale anime series. It's based on a light novel series written by Gen Urobuchi. The anime adaptation was directed by Ei Aoki and produced by Studio Ufotable. It ran for 25 episodes, from October 1, 2011 to June 23, 2012. It served as a prequel to the Fate/stay night visual novel released by Type Moon and written by Kinoko Nasu in 2004, and is part of the prestigous "Nasuverse" which includes the Fate Franchise, Tsukihime, and The Garden of Sinners.

Plot
It is the early 1990's. A war is about to break out. A war between seven Mages who summon and become the Masters of Seven Servants. The Servants are the resurrected spirits of heroes from history and mythology, each placed into one of seven classes and bestowed great power. And they are about to do battle for the greatest treasure of all. The Holy Grail, said to be able to grant any wish, no matter how miraculous, of the one who wields it.

Why It Rocks

 * 1) Incredible animation.
 * 2) Very appealing art regarding character designs, colors, shading, lighting, and backgrounds.
 * 3) Phenomenal action utilizing not only strategy but also the overpowered abilities of each of the Servants and occasionally Masters.
 * 4) Incredibly interesting and complex characters.
 * 5) The show covers great, interesting themes about good, evil, the meaning of justice, and what it takes to be a good King.
 * 6) Great drama which can become downright heartbreaking at times, especially by the end.
 * 7) A story which has a bunch of heroes from history and mythology coming back to life to do battle with each other; it's a fanboys wildest dreams come true!
 * 8) Lots of characters, so you're bound to find at least one that you like.
 * 9) While it does have some one dimensional villains, they still manage to be interesting and entertaining.
 * 10) Manages to be extremely dark and nihilistic without seeming too needlessly edgy.
 * 11) Incredible voice acting in both sub and dub.  The dub in particular is one of the best anime dubs of all time.
 * 12) While technically being a prequel to Fate/stay night, you can watch either anime in either order without loosing out on any enjoyment or getting almost any major plot points spoiled for you.
 * 13) This is one of the few anime that's able to perfectly blend all of the genres that it incorporates, which are action, tragedy, horror, drama, mystery, and psychological, and somehow manages to give all of those aspects the perfect amount of time without having any of them seem to under utilized or too over utilized.
 * 14) Fate/Zero is one of the few anime that has something for just about everyone.  If you want action, they've got it.  If you want drama, you've came to the right place.  If you want tragedy, you can find it here.  If you want interesting, complex themes that you don't find in any type of media, even anime, then look no further. If you want horror, there's enough stuff of it here.  If you want a gigantic cast of interesting characters with none of them being exactly the same, then take a seat.
 * 15) Fans of typical shonen battle anime will be pleased because Fate/Zero has a whole host of abilities, namely the Noble Phantasms, which are incredibly powerful, unique, creative, and personalized for the Heroic Spirit in question.

The Only Bad Quality

 * 1) Because it wasn't written by Kinoko Nasu (or at least assisted by him), the story contains some continuity errors with the original Fate/Stay Night story.

Trivia

 * Ufotable, the studio that animated this series, has been known to produce episodes seasonally rather than weekly, allowing you to have quality-over-quantity shows and the series being easy to finish.