Aliens: Steel Egg
I'm not sure why I was quite so surprised after reading Predator: Forever Midnight by the same author, but this was so bad I almost couldn't finish it. Bits and pieces were vaguely passable, but so much of it is just bizarre and awful, and by the time the heroes are running around with a pink Michelin Man alien called Larry it's just gone completely off the rails. I'm not sure which of Shirley's two novels are worse - probably this, because as shit as Forever Midnight is, its at least mostly about Predators. The Aliens felt like they were an afterthought in this and he really only wanted to write about Larry and his chums.
I thought Alien Harvest was bad, but this is without doubt the worst Alien novel I've read yet.