There have literally been dozens of different Avengers, so if you were in charge, ala a government edict of some kind, mandating who would be in the group, there would be quite a lot of drama in the choice. In fact, long-term fans of the comic-book already recognize that scenario as the main plot of THE AVENGERS #181 (March 1979).
Now, I am going to go out on a limb and say that I am not going to choose any of the super-powerful characters for my Ultimate Avengers line-up. In my opinion, none of the super-powerful characters need to have "team-ups." Sure, in the AVENGERS movie they were fighting an alien invasion. On the other hand, do you really think seven people (six really, not counting Nick Fury) could have stopped it? There should have been cameos by Spider-Man, Daredevil, the X-Men, and the Fantastic Four! But I get off topic. I only want to say that you won't find Thor, The Hulk, or Hercules on this list. Not only because of the power disparancy, but because I don't see these guys as "joiners."
1. CAPTAIN AMERICA
This is the obvious choice, and I don't see how anyone could question his inclusion. In a better movie, he would have had more of the Nick Fury lines and Col Fury would have been a supporting character. He's fast, he's strong, he's smart, and he's got leadership and military training. One of the scenes I really liked in the movie is when Cap (played by Chris Evans) takes it upon himself to order some of the police around, and then, after disposing of an alien or five, walks off while we hear the police chief barking those same orders at his staff. Captain America belongs to and with this group. As Stan Lee himself says, "Nuff Said."
2. IRON MAN
The friendship between Iron Man and Captain America should be the moral and emotional core of The Avengers. Although their friendship has taken some hits during the recent MARVEL event, the terrible "Civil War" storyline, I believe they really are friends. They may move apart temporarily, but there is a respect and affection there that can not be broken. Iron Man is the inventor as well as the powerhouse of the group. Plus he's their moneybags, as well in the personage of Tony Stark. So he should stay.
3. THE WASP
Janet Pym has gone through a lot of changes since her debut in the early Sixties as a partner for Ant-Man. She's now less of an airhead and more of a tactician and warrior. Yet, she still likes to go shopping! In fact, she is famous for never wanting to appear in the same outfit twice. I think this trait makes Jan even more appealing as a character. The Wasp brings power (her laser-like "stings"), intelligence, beauty, and skill to the team. Plus she was one of the original members, a fact that the movie producers shamefully ignored. I would much rather have had her than The Black Widow; sorry!
4. THE SCARLET WITCH and
5. THE VISION
Unlike most of the other characters on this list, Wanda Maximoff and her husband, the Vision, really have nowhere else to go. The Scarlet Witch has a mutant ability to affect things; she calls these blasts of energy her "hexes." She can take down such big-name bad-guys as Ultron or change daisies to roses. She's powerful, but each hex takes its toll on her energy; she can only create so many before she has to rest. Her husband is an android who can change his body to the hardness of a diamond or the wispiness of vapor. They were once one of MARVEL's most popular couples, but John Byrne went and ret-conned some of their history, and now they are mess. If I were in charge, I would re-boot over the last 30 some years and return them as a couple and leave them the hell alone.
6. BLACK PANTHER
Every group needs a stealth character, and in The King of Wakanda the Avengers have one of the best ever created. Also, he's no slouch in the strength and speed departments or in scientific knowledge. I don't want to ask Hank Pym (aka Ant-Man/Goliath/Giant-Man/YellowJacket) to join, so Black Panther can take the scientist role in the group. T'Challa is a very well-rounded character who deserves this spot. And hey, it doesn't hurt that he isn't another Caucasian male.
7. OPEN CHAIR aka TO BE ROTATED
Unlike almost all other super-hero groups, the Avengers seems to thrive on having a rotating membership. The Hulk quit in issue #3. All the remaining original members quit in #16. The Black Widow once joined the group just to quit it in the very next issue! Hawkeye comes back swearing to stay forever, only to get pissed-off about something and quit in a huff. She-Hulk seems to love avenging until the writer leaves and the new guy doesn't want her to stay. Ms. Marvel is everybody's choice for replacement, until she gets a government job and has to leave. And of course, new characters and/or new members are always popping up and in and then out again! Goliath, The Vision, The Beast, Tigra, Firestar, and Triad are just a few of the characters who showed up and stayed!
So my 7th Member is "All of the Above." Personally, I always liked The Falcon. However, in his introduction he was portrayed as not really wanting to be a member, so unless something happens to change that I have to assume he is still not a joiner.
So...who would YOU pick?