I was reading a post on another forum asking "what is the most powerful creature in middle Earth". Well the most powerful creature in the LOTR universe would be Illuvatar, since he is God...but the most powerful creature to ever set foot on Middle Earth? Most would say Morgoth...but I am going with Ungoliant. It seems that Ungoliant had a unique power amongst the other celestial creatures and demons of Arda, she could feed on other power-sources and grow stronger. She was, it can be assumed, weaker than Morgoth when she was first drafted by him to feed upon the trees, and we have to assume this ability to feed on power is a power unique to Ungoliant or Morgoth would have just done it himself. After feeding upon the trees, and perhaps several other power-sources, ungoliant grew in power far beyond Morgoth, so much so that she was able to defeat him in battle in order to consume his silmaril (I think that is why she attacked him) and maybe even consume Morgoth himself. It took the combined might of Morgoth and every Balrog hiding in Middle Earth to drive Ungoliant away, where the only the only thing to finaly defeat her was herself (she ate herself). It also seems that Shelob inherited this ability from Ungoliant since it mentions that Shelob was content to slowly feast over the centuries and grow stronger and stronger.
Since all of the good Gods put together could barley defeat Morgoth, but Ungoliant single-handedly defeated him and fought a Troup or Balrogs.....I think Ungoliant must be the strongest creature in Middle-Earth.