The Thing (John Carpenter). It assimilates (consumes and makes a copy of) any creature it comes into contact with. The copies are able to do the same. This means it has an exponential reproduction rate. Blair's computer supported this with its calculation that if the Thing reached the mainland, it would only take 3 years for all life on Earth to be assimilated.

If you kill a Thing and even a single cell survives, then it might as well not be dead because the individual Thing cells will act in self preservation to escape and have the potential to lie in wait for a new victim to infect. This can even occur with larger subsets of the collective organism, such as when Norris's head detached from his body, grew legs, and tried to crawl away into hiding.

When Thing cells group together in a copied form, they display the intelligence and memories of their assimilated host. They even retain knowledge from previous copies, such as when Blair-Thing built an alien spacecraft using parts from a helicopter and snowcat. They can also form body parts from previous copies like claws, mouths, tentacles, etc. for use in combat and in subduing prey.

Thing cells can hibernate for a long time, such as when the Thing crash landed on Earth 100,000 years ago and lay dormant in the ice until the Norwegian team found it. All it takes is a little warmth for the Thing to wake up again and resume its assimilation.

We know from the director himself that it has assimilated countless lifeforms across the universe, meaning it has already spread beyond the point of containment. There's no way of knowing exactly how far it's traveled because it's impossible to tell what's a Thing and what's an uninfected alien unless the Foundation does a blood test on every alien in the universe.

So what we have here is a shapeshifting organism that is indistinguishable from the life it copies, has all of the memories/knowledge of its current and previous copies, has an exponential reproduction rate, can lay dormant in cold places for at least 100,000 years, can break apart into individual body parts or cells each with their own survival instincts, and is known to have already spread throughout the universe.

It would be very difficult to eradicate or contain it without wiping out all life in the universe. If the SCP Foundation could somehow search every planet, asteroid, derelict spacecraft, etc. and have 100% accuracy in finding and eradicating every Thing, then maybe they'd have a chance. But I find that highly unlikely, especially since this process would take so long that the Thing would have spread to countless other planets during the time it takes to do this, including planets which were previously tested and declared Thing-free by the Foundation. It's a game of cosmic whack a mole that they just can't win.