Those two are tied for me. I see that Aliens is a better movie, better characters, acting, plot but it loses some points due to the introduction of the "bug hive thing" since I liked the portrayal of the original creature more.
While on Predator 2, it took the stuff I liked about the Predator creature on the original and pumped some gains on it, more Predator moments, kills, weapons, a look on a ship and its clan, without changing the views I had on it.