And yet, when you think about it, only about as smart as Inspector Rex.
Difference being, as has been pointed out, is that Aliens are much, much faster learners.
I'd personally like to see Aliens that have been alive longer than three weeks in an environment with minimal external influence. All Aliens we've ever seen have been completely separate from whatever their origins were; it's unfair to assume we've seen the pinnacle of Alien intelligence based on 3-week-old babies.
Raise a person outside of society, in the wild, and they will not be molecular biologists. They won't even know what the hell a molecule is. The only reason we're so smart is because we've had two million years of other people doing the groundwork and passing it on down. You don't get that when you're born in a vacuum.