This is a big problem because I think they ran out of good ideas a long time ago. The last one that struck me as any good was the graboid in Tremors, but that film is pretty old now. The film Alien really turned the whole genre around and most stuff that's come afterward seems to have it's roots in that film, one way or another. Mimic and Species, which both had good sci-fi monsters, still reminded me alot of Alien. I think we need a new sci-fi monster that comes from a different direction.