The Energon Pub

Okay, to be specific, this is the Thing from John Carpenter's "The Thing" (and it's prequel of the same name), versus a xenomorph from the various Alien movies.

For the title of most murderous, terrifying alien in movie history!

The Thing, obviously. For all of the built-in weapons a Xenomorph has, none of them match up to the assimilation abilities of the Thing. We're talking about a being whose every cell is an individual being, and can devour and replicate the cells of any living being around it, perfectly mimicking it until it wants to kill or spread itself.

And seriously, a monster with a giant mouth where its torso used to be, with bones for teeth and razor-sharp tentacles whipping around everywhere, cutting everything they touch, but a human face still screaming at you. What's more terrifying than that?

The Thing would get horribly burnt by the Xenomorph acid blood. It would win, I suppose, but it would have to spend a lot of time recovering.