Seven
Assignation of teams after graduation was arranged by the Hokage. It gave chances for a team to develop their areas of specialization according to their potential. Teams 1-6 were generally all-purpose and most end up as career chunins and some made it to jounin or even ANBU. Team 8 was always a tracking and reconnaissance team, team 9 an infallible escort team, team 10 a capture and interrogation team and team 7 was made for heavy combat.
Team 8 would always need an Inuzuka for their heightened sense of smell, an Aburame for their kikaichu or a Hyuuga for the Byakugan. It was lucky that they got all three. Team 9 always had a Hyuuga and a ninjutsu or taijutsu expert and a weapons specialist to complete the team but there was no team 9 that year. Team 10 was always composed of someone with the brains for strategy, a trap expert or anyone with means for binding, and someone skilled with taijutsu or ninjutsu for defense but it was made famous by the previous Ino-Shika-Cho combination hence it was repeated in their year. Team 7 was always composed of the top rookie, the top kunoichi and the dead-last. It was supposed to be well-rounded in taijutsu, ninjutsu and genjutsu with additional proficiencies in kenjutsu, fuinjutsu and iryojutsu. They were supposed to be a hard-hitting team and Sakura didn't know why she was included in the first place except perhaps for her brains and chakra control and to be near Sasuke-kun.
Sakura would later on learn that their team in itself was a metaphor. Seven was supposed to be a lucky number but so far all of the previous team sevens were ill-fated. Seven was a number for betrayal, for bad luck, for a team set-up for greatness yet doomed to fail. Like all other teams, history tends to repeat itself.
The Sannins taught by the Sandaime were the earliest example. Jiraiya, Tsunade and Orochimaru grew up during the times of the Shinobi war. They were hardened by battle and their losses. Jiraiya and Orochimaru were proficient with taijutsu and ninjutsu. Orochimaru was the only one skilled in genjutsu though Jiraiya did well in fuinjutsu. Tsunade used her chakra enhanced strength for taijutsu and if not for her unparalleled skills in iryojutsu, both her teammates would have died many times. They exhibited the ideal of team seven for teamwork but sadly it didn't last.
Orochimaru would betray them when he didn't get the seat for Hokage but he has already fallen way before that with his illicit experiments and quest for immortality. Tsunade heartbroken from the loss of her brother and her lover would leave the village her family helped built. And Jiraiya would live a life as a nomad gathering intel for his village but never staying long enough. The village reminds him too much of his failure to keep his teammates, of the man who has descended too far in depravity to be saved and the woman who'll always have his heart despite his lecherous ways. The only time they were reunited again was when they fought 2:1 versus Orochimaru, former teammates turned to kill each other.
Then there was Jiraiya who taught the future Yondaime. Namikaze Minato was an orphan but he has erased all preconceptions by turning out to be the top of the class. He was a student worth teaching and had a humble attitude and easy charm. Minato's brilliance however, unintentionally was not meant for a lifetime confined in a cell of three, not that his teammates were incompetent. He was just meant for greater things that he could accomplish alone, the Hiraishin, the rasengan, the Hokage seat and sealing the Kyuubi.
And so there wasn't much on record about their team and the names of his teammates had been forgotten. It wasn't his fault he would outshine his other teammates but nonetheless it was another bad mark against team 7.
Minato would then become sensei to Hatake Kakashi, Nohara Rin and Uchiha Obito. There was much hope in the selection of this team, an unlikely Uchiha who dreamt of becoming the Hokage, a cheerful girl aspiring to be a mednin and a stoic prodigy with a tragic past. But maybe they hoped too much too soon. Obito and Kakashi would fight just about anything, insulting each other and taking their spars a little too seriously. Rin would always try to intervene and patch them up afterwards. Kakashi was too much of a brat back in the day in proving he was the best in everything, belittling Obito and sparsely gave attention to Rin. He was much too focused on proving he was not like his father. Obito had his heart in the right place even if he was deemed to be un-Uchiha like especially with his lack of their famed doujutsu. Rin loved both her teammates but she would always have a preference for Kakashi who was indifferent, to the annoyance of Obito who has come to like Rin. It wasn't until that mission against Iwa when they cemented their teamwork and finally acted like a team. But by then it was too late.
Kakashi lost an eye when they went to rescue Rin and so was at a disadvantage. Obito saved him when he had awakened his kekkei genkai but he ended up crushed under a pile of rocks. His gift to Kakashi would be the Sharingan which will later on make Kakashi infamous as the copy-nin. Rin did the transplant and even then it was amazing how she accomplished it in such setting. It was the start of the demise of another team 7.
Hatake Kakashi would grow hardened and cynical and would not find any team passable to become team 7, not until they came along. Uzumaki Naruto looked like his father but acted like his mother. He was the Kyuubi vessel, as dangerous as an exploding tag but was still naive of what being a shinobi entails. Haruno Sakura was such a girl, with her one-track mind of the Uchiha if it weren't for the reports of her intelligence and chakra control. It could be because she was from a civilian background and did not know any better. Uchiha Sasuke was the top rookie of their class and the sole survivor of his clan's massacre. It was a forewarning already that a past like his could only produce someone troubled but he gave him the benefit of the doubt. They had abysmal teamwork as proven by the bell test and their only saving grace was the fact that they did not leave each other to hunger. He figured it was good enough to pass them by Obito's standards.
In time he would see how they mirror his old team, how Sasuke and Naruto would always squabble and Sakura would interfere in favour of Sasuke; even their love triangle was similar. Obito was Naruto, Rin was Sakura and he saw in himself in Sasuke. But he wasn't like his own sensei, so he spent more time trying to guide Sasuke to the right path at the expense of Naruto and Sakura, even teaching him the chidori which he told him was to be used to protect those he loved but it became more difficult and more obvious that he was set on becoming an avenger after he got the cursed seal. The final nail to the coffin of team 7 was when Sasuke left to join Orochimaru and Naruto and the others came back wounded and defeated. Sakura was inconsolable for days and Naruto took it as a personal failure and vowed to bring Sasuke back home. Kakashi felt as if he lost his original team all over again.
Naruto would leave and train with Jiraiya while Sakura would study under Tsunade who was now the Godaime. All his students were now apprentices of Sannins and maybe it was their destiny. As team 7 had the history of repeating itself. They would reform again after almost 3 years and be named team Kakashi. Somebody must have realized that 7 was cursed, even the newly graduated genins had teams named after the alphabet. They would gain 2 members, Yamato who is captain in his absence and Sai, who is not quite a replacement but someone to fill-in the vacancy. The original team 7 would meet each other again and again and one of them is always trying to kill the other.
Now, they were facing another war, and team 7 was reunited once again. Although this time they were fighting on the same side. It felt right once again to be fighting against a common enemy together as a team. Maybe this time, they could finally show some teamwork. For how long, who knows? But it isn't inconceivable to hope that finally this team 7 can redeem itself.