While they normally purr, when they see a Klingons they scream (After the Klingon "The Great Tribble Hunt")

Why they're tough: While they look cute, they are born pregnant, and reproduce at an exponential rate. Once they eat all the food, they turn to eating human flesh.

Not sure if having both a sense of humor and dress is a strongpoint...

Why they're tough: Sneak attacks...They start off cute, get the midnight munchies and then attack. Not to mention they multiply by water whenever they happen to run across it. (thankfully it only covers 75% of the Earth)

When will people learn not to poke alien larva with a stick

Why they're tough: They swarm with a single mind and they try making you swallow.

Hunted by bounty hunters just for existing. They found food in Kansas, LA, and space.

Why they're tough: They roll in packs, shoot poisonous spikes, and have rows of teeth to eat humans with.

Eating most of the people in Nevada, they are heading to Vegas next.

Why they're tough: They attack from underground, have snake tongues, and sense movement by vibrations.

While we aren't what they are or where they came from, we do know they kicked the Statue of Liberty's ass.

Why they're/it's tough: Can destroy NYC, survive bombings, and they find time to drop of mini-versions of itself that make people explode.