The head redesign in Aliens is what kills that one for me. With the bare sockets they don't look alive, just mechanical. Having the 'eye' sockets on the side of the head means they don't even look predatory. And losing that sheath means they don't look as penisy, which kind of defeats the whole point.
If they'd just kept the dome, it might have been my favourite design. Like in Alien 3, the mouthparts have a lot of articulation and expression, and other than the head it keeps most of what made the original design great.