It's who's the most iconic, not who actually has the most appearances. Anyone from the original or Kooky Quartet roster except Hulk would fit that description (Hulk's an iconic character, but not as an Avenger, since he left in issue 2). Most Avengers fans know the Iron Man, Thor, Giant-Man, Wasp, Captain America and Captain America, Hawkeye, Scarlet Witch, Quicksilver rosters. That's why Quicksilver qualified. And once you consider all of them, plus Vision, only one more slot is left.
Of course in the 90s Pietro became associated with the X-Men instead, being a member of X-Factor, which is why the X-Men movies ended up using him, but not Wanda who stuck to the Avengers until Disassembled.
A lot of fans cried foul when Black Widow and not Wasp was a founder in the movies. Before then, she wasn't an iconic Avenger, in fact she's associated with the team's worst era, the mid-90s just prior to Heroes Reborn (you know, the period with teenage Iron Man and mutated Wasp).