Would be a tie between the predalien and the upgrade predator.
The predalien just looks wrong. The yellow color, the dreadlocks. I can accept the fleshy design of it, as the other AVP aliens all look more fleshy than biomechanic, so it's consistent. You can no doubt design a great version of the concept , but it's just so off in the movie. At least for me.
Newborn was alright. It fitted with the tone of Resurrection and it was supposed to be an abomination.
I liked the neomorphs in the prequels. They looked like an unfinished work in progress on the way to the real deal. Like tadpoles to a frog.
The super predators where ok i guess. I did not like them very much, but they where not offensive to me. Would have prefered if they toned the face of the berserker down a little bit. I din't like the "superior" predator angle, not sure if it was in the movie at all but it was in one of the scripts for sure. Just have them as a cousin race or a rouge faction.
The upgrade predator is horrible. Its looks like a predator crossed with a inbred ogre. If you really wanted an genetically upgraded predator, give him better senses, make him faster, give him the ability to regenerate his body, better reflexes...anything but just turning it into an hulking brute.
It's like the first Iron Man movie when Tony Stark fought the bigger Iron Man suit in the end or the Hulk movie when Hulk fought the bigger Hulk in the end or... the list goes on endlessly. Im just not a fan of taking the old thing and just making it bigger. You have so much ways to make it more dangerous than the old thing. Just making it bigger is the laziest one. Also not a fan of the implanted armor, weapons and vision mode contact lenses of the upgrade. The nanotech stuff is just too much Iron Man/Marvel/comic book for my taste.
Oh btw not really a creature in the truest sense of the word, but the worst of all is the predator killer in the current predator movie.