Source: comicvine.com

2. Avengers #4 Team

Yes, the Avengers technically had a team in the three issues before this one, but the team didn’t fully solidify until Captain America showed up. And then they had the ultimate Avengers lineup that would define all the others.

Captain America. Iron Man. Thor. Giant Man. The Wasp. The definitive Avengers team.

Too simplistic to be the best, but defined just enough to be close. There’s no weak link on the team. This very squad would go unamended until they were all switched out twelve issues later for the previously discussed Kooky Quartet version. But they’d remain as the founders (asterisk needed) of the rest of the series and throughout all of time.

The Avengers existed without Captain America, but in much the same way that Batman existed before his no-killing rule did: in an unfinished state. Captain America joining the Avengers is what truly made them The Avengers.