Let’s get the exposition out of the way.
Looking for the best, most successful, or just your own personal favorite, team character.

By that I mean, a character created specifically to join an existing team.
In some cases, whole teams of new characters are introduced all at once. That’s not what I’m asking about (though it’s an interesting idea for a different thread, actually).
Not looking for pre-existing characters who join and come to epitomize the team (like Cap/Avengers, Wolverine/X-Men), but original characters that debut w. the purpose of becoming team members.

The obligatory example:

The Vision debuted in Avengers V1 57, and joined in Avengers V1 58. He pretty much the ur example for this type of character. He’s been in some solo books/minis, but easily 90% of his history plays out as an Avenger.
He’s been in a couple of Avengers cartoons, made it to the MCU, and is going to costar in a web series this year.
Not too shabby.