First the big ones: Cowboy Bebop (カウボーイビバップ) and Samurai Champloo (サムライチャンプルー) are both stylish and fun, with some incredible music. And while almost every anime fan will profess to enjoy them effusively, the stories are ultimately too episodic to be life-changing.
Fullmetal Alchemist (鋼の錬金術師・Hagane no Renkinjutsu-shi) is utterly superb. But since neither of the two anime series is definitive (the 2003 version was completed long before the manga, offering an effective but very different conclusion), it’s hard to say you’ll be missing out if you haven’t seen one or both, as each version suffers slightly from the bits of the other that it lacks.
Steins;Gate (シュタインズ・ゲート) offers clever character development wound through a Rubik's cube of a time loop. Slow to get started, it suffers from the fact that resolving its crises ultimately means undoing nearly everything interesting that happens. The series tips into the “essential” category only for card-carrying otaku, as its immersion in Akihabara and cosplay culture has made it an otaku touchstone.
Code Geass: Lelouch of the Rebellion (コードギアス 反逆のルルーシュ・Kodo Giasu: Hangyaku no Rurushu) is a fun Übermensch story—to the end of season 1. But the scenario reset and increasingly implausible plot devices of season 2 ultimately drop it down to the “optional” category, despite the kicker ending.
Death Note (デスノート) is another Übermensch tale, but without any of Code Geass’ fun. Unrelentingly grim and morally rigid, it resonates best if you’re a young teenager angry at the world. If you want a primer on the concept and characters, the 2006 live-action movies effectively distill the story, skipping most of the anime’s second and third acts to jump to the conclusion with measured efficiency.
Every one of Mamoru Hosoda’s films is delightful. Among them, The Girl Who Leapt Through Time (時をかける少女・Toki wo Kakeru Shojo) will warm your soul, Summer Wars (サマーウォーズ) is infectious, and Wolf Children (おおかみこどもの雨と雪・Okami Kodomo no Ame to Yuki) is heart-rending, particularly if you’ve lived in countryside Japan. All are brilliant viewing if you’ve finished your Miyazaki and are desperate for something similar. But as lovely as they are, they’re not quite “necessary.” Perhaps Hosoda’s next work, Mirai, slated for 2018, will push him over that edge.
One Piece (ワンピース) is great, but because it’s so preposterously long (more than 800 episodes and counting since its 1999 kick-off), there’s no “essential” sequence to watch (though people certainly have their opinions), and no sense that you’ve missed something life-changing if you don’t see one bit or another, since the next arc may make the preceding struggle pale by comparison.
Another big name, Gundam, has had a perennial following since its inception in 1979. But with so many variant series (around 20 not including OVAs and movies), as well as entirely alternate universes, it’s hard to call any one of them “the one” to watch. Otaku should know who Char Aznable is and what a Zaku looks like, but beyond that, tuning into a Gundam series is a matter of personal choice. Those interested can find detailed recommendations here.
Yet another classic, Rurouni Kenshin (るろうに剣心), presents a gripping story up to the end of the Kyoto Arc (episode 62), after which it devolves into filler. The first four-episode OVA series offers precedent-setting animation and music, which might in itself justify inclusion on the “essential” list, but unfortunately the series’ muddled ending gets in the way: the second OVA attempts a conclusion, but diverges disturbingly from the manga; and while the manga ending is much more measured, it’s still not as strong as the middle segments of the story, leaving Kenshin a cut shy of required viewing.
Finally, the 1997 Berserk (ベルセルク) anime is a gut-punch, making brilliant use of music to compensate for fairly basic animation. But since it only represents a tiny snippet of a manga that now feels like it may never end, it’s hard to say it’s required. The movies aren’t bad (mostly retelling the 1997 series), but you’d be much better off reading the manga than bothering with the misguided 2016 anime revival, which picks up from the end of the movies. Berserk is also significantly more brutal than even Attack on Titan, making it hard to recommend for the average viewer.
That's it! Think we missed something? Disagree? Let us know in the comments below!