The strongest creature created by Morgoth does not exist. He could not create new individuals as could not all the Ainur. However Morgoth could not create even his own 'mind beings' like Aule did the Dwarves. By this I mean he could not distribute his own mind into bodies of others thus making beings which were utterly answerable to him. For some reason, unlike his brethen, he lost that power after descending into Ea.

The Mightiest of Morgoths servants is Gorthaur the cruel/Sauron.

The most terrifying is the Valaraukar, the Balrogs. Which would win in a display of Might, Gothmog or Sauron is hard to tell.

The next would be Dragons, though they are far less mighty than Balrogs. Greatest of these is the Great worm Glaurung - father of Dragons, and the the winged dragon Ancalagon the black.

Ungoliant was only servant to Melkor for a short time and her origins areunknown but she may have therefore been powerfuller then Sauron and the Balrogs. All we know of her power is that it was not superior to Morgoth's.

"Now those drakes and worms are the evillest creatures that
Melko has made, and the most uncouth, yet of all are they the
most powerful, save it be the Balrogs only."
Book of lost tales 1

"The Shadow that bred them can only mock, it cannot make real new things of its own. I don't think it gave life to the Orcs, it only ruined them and twisted them."
Lord of the Rings

"Among those of his servants that have names the greatest was that spirit whom the Eldar called Sauron, or Gorthaur the Cruel."
The silmarillion

^
yes I agree (dunno if I have posted before.. *mind loss*) but I think the same..

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