Alien by a huge margin.
The original is one of my favorite films of all time, and holds up oh so well as time goes on. It's just plain art. Aliens was also amazing, if slightly less timeless. And I'll gladly stand up and defend Resurrection and Prometheus to anyone who wants to actually discuss them. Even 3 and Covenant, while not great, have their good points and are at least watchable. Alien also has an undeniably larger legacy in both pop culture and film history.
Predator was a great stand alone. Prey was good. Everything else is camp at best. Sometimes fun, for sure, but not "good" in most ways that count.
I'll also throw out there that AvP was a fun romp, but as a whole that franchise is better suited to video games and comics. It shouldn't be canon for either bases.
As a late note, I'll also posit that both Alien and Aliens are cases where theatrical cut is actually better than the director's cut. While I have seen and appreciate all the deleted scenes, the stories are better without the added stuff.