I think what really elevates Aliens is the emotional core provided by Ripley and Newt (and Hicks to a lesser degree). To me, Aliens has the strongest emotional impact out of the franchise...I really feel Ripley's emotional anguish when she loses Newt (keeping in mind the aging and death of her biological daughter), before she resolves to make a rescue attempt into the hive. Ripley was already a great protagonist in the first film...I'd argue that Cameron's take on Ripley is arguably the most memorable, badass (but oh so skilfully humanized) heroine in cinema history.
Alien's strength is the cosmic dread and keeping the nature of the titular Alien (and the Pilot) very mysterious, very...alien, and one might say Lovecraftian. I'd argue that Cameron's take does not necessarily reduce the Aliens into giant space-ants, but that could be a more simplistic interpretation of Cameron's portrayal, and of course the Aliens and AvP EU really ran with the "bug" idea.
At the end of the day, it has to come down to Alien or Aliens. It's like comparing a good apple vs. a good orange. I'd give Aliens the edge based on its, in my view, superior emotional resonance and character relationships.