Sometimes, an anime just needs a solid revamp, and the best anime reboots stand as a testament to the power of re-imagining a beloved series. This practice, where an old anime gets reworked with modern animation and fresh ideas, injects new life into a series either forgotten by time or without a proper conclusion. Some of the best anime remakes introduce anime fans to history they might otherwise have ignored. That's what happened to Devilman, a '70s anime remade in the form of Devilman Crybaby, which updated the art and style and introduced new fans to the series.
Another reason a studio may remake an anime is to more closely follow the manga source material. The 2003 version of Fullmetal Alchemist boasts plenty of its own merits, but it largely veers off course from Hiromu Arakawa's story in the manga. For fans of the manga, the 2009 remake, Fullmetal Alchemist: Brotherhood, is a welcome addition to the anime canon, even ranking as one of the best anime of all time.
Not every anime that deserves a reboot receives one, and some reboots even end up eliminating the joy of the original series. That said, the anime below not only rebooted their series faithfully, they did so in ways that improved upon the existing property. This doesn't necessarily mean that the original series is bad – instead, it means the reboot adds something new, and provides a worthy viewing experience all its own.
The original Hunter X Hunter started on some solid footing, but it ended up on hiatus more than once due to the source manga's erratic update schedule. After the first series began in 1999, production stopped just short of the Chimera Ant arc, as there wasn't enough material available and they wanted to avoid more filler. Manga updates came so slowly and sporadically that it took until 2011 to gather enough material for a complete arc.
Rather than creating the arc and risk going through the same thing again, production decided to restart the series from scratch, paying more attention to character development and faithfulness to the source material.
Everyone has an opinion on which version of Fullmetal Alchemist is better, which means the anime community may never agree on an answer. Both have their merits, but only the reboot, FMA: Brotherhood, truly remains faithful the manga storyline, which is a fantastic story on its own.
While plenty of amazing anime excel without strictly adhering to their source manga, it's a blessing at least one version of Fullmetal Alchemist brings manga creator Hiromu Arakawa's original ideas to life.
The main thing differentiating Hellsing Ultimate from its predecessor Hellsing is its loyalty to the manga storyline. To be fair to the first series, it couldn't follow the manga because the manga wasn't anywhere close to being finished. In order to create an anime with a coherent storyline, the anime creators came up with their own villain, a vampire named Incognito.
Hellsing Ultimate follows the manga closely, and is generally considered to be the superior version of the Hellsing story. In addition to telling a more compelling narrative, Hellsing Ultimate boasts much better animation.
Fate/Stay Night is based on a visual novel, where plots and endings are determined by the reader's choices. An anime adaptation can unfortunately only choose one ending, which means fans of the video game may not see their favorite outcome unfold on screen. This problem is ameliorated with the rebooted version of the story, Fate/Stay Night: Unlimited Blade Works. While protagonist Shirou becomes romantically involved with his partner Saber in the first series, in the second series he instead becomes involved with Rin.
While there are merits to both romantic outcomes, Fate/Stay Night: Unlimited Blade Works, which was produced by ufotable, offers a significant improvement in art, animation, and special effects over the original Studio Deen version.