Thanos, Ultron or Kang.

Namor

Either Doctor Doom, who wants to rule the universe, or Galactus, who wants to eat planets.

Because they feel like actual threats, rather than someone who just wants to cause chaos.

Ultron.

I would've said doom. But he's not exclusively an avengers villain. He's mostly an FF villain, who goes after the avengers, on occasions.

Ultron, for me.

Same with Galactus he’s also an FF villain, but also chases the Avengers on certain occasions.

Well, he's kind of a universal villain. Who first appeared in an FF story. He doesn't really give a fuck about FF, as far as I know.