My favourite is probably the model from Aliens, although the Big Chap's barely visible skull is creepy as hell and seems more visually effective for a stalker / ambush style creature, than the swarms of melee-oriented creatures in the sequel.
Part of that is habituation to where the two styles appeared over the years, sure -- but my logic is that the creepy detail lends itself more to horror and suspense as in the first film, and would look played out pretty fast if we saw it over and over again in a swarm.
To be honest, I don't remember if it was a detail we saw clearly in Alien, been a while since I've watched it.
The Dog Alien from Alien 3 was cool enough, and although the movie was goofy I didn't so much mind the standard warrior Xenos in Alien: Resurrection; that being said it seems every rendition of the creature after the first two films was some kind of step down to varying degrees. The tendency in film to re-design and over-design seldom lands, IMO.