Natasha Romanoff is very likely the Marvel Universe’s most dangerous woman on Earth. Schooled in a hundred fighting styles, the tradecraft of a one-time super power nation and an agent of SHIELD, Natasha played the most dangerous game on both sides of the Iron Curtain for years before she put on a costume and started fighting guys with undersea bases and death rays. She’s in my Avengers for her level head, command ability (she’s the second in command after Cap as far as I’m concerned), nearly unequaled combat experience and she’s the leader of the Avengers Espionage Squad. This is an idea that I stole from the Legion of Superheroes and it seems to make sense in the Marvel Universe even more than the DCU. If you live in a world with SHIELD, Hydra, AIM and all the other super-espionage outfits the MU boasts, you are periodically going to need to infiltrate, sabotage and, frankly, spy on a bunch of organizations. Years of working both sides of the Cold War makes Natasha more than capable of leading and coordinating the efforts of the Espionage Squad and taking over the Avengers’ strategy if Captain America should ever be unavailable.
During our JLA discussions, Tate noted that our choices demonstrated the difference in the sort of JLA stories each of us were wanting; I think that same idea holds even more weight with The Avengers, especially since there's not a single point of agreement between us this time around. His idea of an Avengers Espionage Squad, while interesting, doesn't mesh with my mental picture of the team; this shadowy Avengers feels very Bendis to me, and I always preferred my Avengers to be a bright and shining a la the Starlin, Shooter, and Stern years. Although I know Natasha served a lengthy term on the team, much like Spider-man I've always thought she functions much better as a solo agent.
Out of all of the characters on my list, I don't thing any of them scream "Avenger!" to me as much as Ms. Wanda Maximof. The Avengers were where she went to seek redemption after her time following Magneto; the Avengers were where she learned to tap into her mystical heritage; the Avengers were where she met the love of her life; the Avengers were where she went insane and murderous . . .
Er, um, scratch that last one, okay?
Seriously, though, Wanda seemed to latch onto the "Avengers as family" theme more strongly than most anyone else; add onto that the versatility and coolness factor of her powers (nebulous though they may be), not to mention my early fascination with her (more on that soon) and that's why the Scarlet Witch is a must-have member of my team.
I think you can compare Wanda with my Vision response in many ways. She's just wrapped up in so much stupid stuff! House of M was the absolute capper and it will be many years before she stops being radioactive for me. Aside from the general distaste I have for what's going on with her, I can't argue with the Avengers-as-family feel for Wanda or with the potential coolness of her powers (although that statement strays dangerously near to radiation levels unkind to human life).
I did like her quite a bit during the oft-mentioned Buskiek-Perez run, though. Not enough to wash House of M and Disassembled out of my mind, though. I know I'm the one who keeps bringing up the hotness factor for membership, but I think it needs to be brought up one more time so I can talk about George Perez. Perez managed to give Wanda a costume that, while a little more ridiculous than standard super-hero fare, managed to show off her gypsy heritage and her sexiness. Perez also gave her a slightly different look in her facial features that showed she was a different ethnicity than her teammates without pounding us over the head with it. When you looked at Perez's Scarlet Witch, she looked like a hot magic-user of gypsy descent. Her costume told the tale and accentuated the hotness, you can't go wrong with that.