It's become almost impossible to find a shonen anime series that does not follow the trend of having a team or group of super deadly foes to oppose the protagonist(s). One villain is enough. So when a group of these baddies get together, it's usually no fun for the heroes. In contrast, it's very entertaining for the viewer. Most of these villainous groups or organizations, are filled with unique individuals who have various different backgrounds and motives.
The Espada are the 10 strongest Arrancar chosen by Sosuke Aizen in order to defend Las Noches (his fortress) and to accomplish his plan of defeating the 13 Gotei; in particular, the Espada are hollow-shinigami hybrid and, for this reason, they have a more human-like appearance than the others Arrancar.
They all have different abilities and they can also improve them thanks to their Zanpakuto release, but, if individually they're incredibly strong, as a group they're not because each one of them is convinced to be enough strong to defeat any enemy on themselves and, for this reason, they are used to underestimate their opponents.
Among the strongest Arrancar there are Barragan (who can decompose everything he touches), Ulquiorra (who has an incredible strength comparable to Ichigo's one) and Grimmjow.
AKATSUKI - NARUTO
The Akatsuki was originally created by Yahiko in order to stop all the wars and bring peace to the world, but after his death, his friend Nagato realized that the world couldn't reach peace by itself, so that he started working in order to bring the "true peace" to the world, which means creating a fictitious world in which everybody can be happy with their lives thanks to an illusion.
Although in Akatsuki there are ninjas coming from different villages and nations, they are used to work in couples, so that they have a (small) concept of "working together".
Among this group, we recognize very incredibly powerful characters, such as Itachi (the master of Illusions), Pain (the user of Rinnegan), Hidan (the immortal shinobi) and Tobi (the intangible masked man).
PHANTOM TROUPE - HUNTER X HUNTER
The Phantom Troupe, as known as "Spider", is a group composed of 13 expert Nen users; all of them are thieves known all over the world and ranked as A-Class criminals. If someone wanna become a member, then they have to kill a current member. If a member dies, the leader has to find a new member, so that the total number of members will always be 13.
They're able to fight against the Zoldyck family without being outclassed; their only weakness is the fact that, in many cases, the members hate each other, so that they are not able to cooperate at their 100% in a serious fight.
Among the strongest members we remember Chrollo (who can steal other people's powers and abilities), Hisoka, Feitan (the cold-blooded killer) and Nobunaga (one of the best swordsmen ever).
SHICHIBUKAI - ONE PIECE
The Shichibukai is a group composed by seven incredibly powerful pirates who accepted to ally with the World Government and work for it.
They all possess incredible abilities, both physical and also coming from Devil Fruits powers; they're not a proper crew, since members of Shichibukai are often lonely pirates or, on the contrary, captains of their own crews, so that they don't need to take part to another crew. For this reason, some of them can't cooperate with some others because they don't get on well each other and they don't form a solid group.
Anyway, among Shichibukai we can find some of the strongest One Piece characters ever, such as Mihawk (the strongest swordsman in the world), Blackbeard (the man who owns two Devil Fruits powers), Doflamingo and Kuma (whose powers have been used to create weapons for the World Government).
AOGIRI TREE - TOKYO GHOUL
The Aogiri Tree is an organization that has been created in order to change the world and build a society in which Ghoul and humans could live in peace; it's composed by incredibly strong Ghoul who developed their powers during numerous fights.
As we saw in the anime, they can cooperate really well and they're able to organize their attacks perfectly, but in the end each one of them seems to fight for their personal interests.
Among the strongest characters we find, of course, Kaneki (after the Centipede awakening), Eto (the One-Eyed Owl), Noro (who seems intangible and immortal, but in reality he only has incredible regeneration skills), the Bin Brothers (who are not strong if you take into consideration each one of them, but when they fight together they're lethal thanks to their combinations and attack schemes) and Tatara.