Aliens.
Two reasons.
1. In general I think the Alien films are better(excludes resurrection, because it was awful, alien 3 sucked too, but not s much)
2. it's the design of the Alien. It's a much more terrifying creature than a predator, because it simply is more alien. Predator has somewhat humanoid features, and you can see its emotions to an extent. The aliens on the other hand, are basically wild animals that need to survive.
Even Giger himself said that he liked that the Alien has no visible eyes, as this gets rid of any human-like quality, and gives him them the appearance of an emotionless mechanical killer.
The Alien's design was just so shocking and full of impact, I just think it's the most unique design for an creature I've ever seen.