Yes and no, the bodycount in the end is always in favor of the aliens (compared to human casualties). But the aliens are very alike normal earth predators, making them not so special. You might just as well vote for the sharks on Discovery Channel, regarding aliens. The movie Predators are very sophisticated and ritual killers, thereby deserving more credit in a poll like this.
But if it weren't for the predators shit they wouldn't be the bad assed killing machiens.
Aliens spit acid,bleed acid,head bite,pounce,wall walk,and can see(smell,hear what ever) everything,Aliens' agility is the most distinctive characteristic about the alien(besides the slime).
now tell me one thing that the predators has over the aliens that they had at birth.
Ok, I was a bit blunt at my previous comment, and did indeed not pay the rightious respect to the aliens. But this is about being the best killer. The origin does not matter, it is the final caracter that counts. And that includes all the training and equiptment a Predator drags around. Or else we would have to post Jason as a 8 year old bald boy on this, right?
And sure a alien tops off any fresh predator, but that is not the point.