Rating the impact of monsters is pretty well impossible. Consider dragons and drow, then think of orcs and giants. Impossible to determine which are more meaningful in the many, many campaigns that have been run, are being consucted now.
I enjoy playing the part of any challenging monster or NPC, making the players feel their PCs' are in dire peril, for that is entertainment. Of course as a player I am anxious when faced by such a threat to my PC
About the worst monster I faced with Rob's iron golem that could levitate, breathe fire, and was armed with a whip causing petrifaction and a poisoned sword. that critter did for both Mordenkainen and bigby, and only a wish restored things
Two outstanding mosters in my memory of past adventures with me as the DM are the pair of black dragons that were loosed by the PCs on a level of Greyhawk Castle and Obmi the dwarf NPC that all the chaps got to really hate
Cheers,
Gary
Cheers,
Gary