On the Predator side I have to say that I love the original Pred from the first movie. Not only the best looking of the lot, (only a cosmetic attribute, but damn did it look badass, lol) but in my opinion, the best hunter of them all. The way it effortlessly stalked it's prey & waited for the perfect moment to strike was awesome. The originator of the series really set the benchmark to which all other Predators from then on have been compared, & in my opinion, the only one to have come close was the second Pred, which is high-up on my list as well.
As far as Aliens go, it's quite close for me between the original Giger Drone, & James Cameron's Warriors. The Giger Drone was the originator of the series, the creature which has terrified audiences for 26 years, the creature which many claimed "made the shark in Jaws look like a goldfish by comparison". The Warriors on the other hand took the creatures one step further. Larger, faster, & stronger than their drone counterparts, they were an undeniably brilliant creation. In all honesty, I have to say I prefer the Warriors overall, they are fantastic creatures, & my personal favourite Alien form.