The shape shifting Skrulls most recently battled The Avengers in the Secret Invasion storyline. Years of planning (and switching out key members of Earth's Mightiest Heroes with their own people) led to them attempting to conquer Earth, and while they may have failed, the aliens were still partially responsible for Norman Osborn's rise to power. However, this lot have been trying to destroy The Avengers for decades! Perhaps the most notable example of that was during the Kree/Skrull War. It was during this battle that the Skrulls - who had almost solely battled the Fantastic Four up until this point - really showed how evil and ruthless a race they are, detonating a bomb and committing genocide by murdering millions of innocent Kree people in the process. Of all the aliens taken on by The Avengers, the Skrulls are the most formidable.