Relative Power of the Bad Guys?
 

Well, we know that Melkor was the most powerful evil being. But once he was weakened by the slaying of the Two Trees, Ungoliant almost "killed" him. If Melkor was originally the most powerful Vala, is Ungoliant of Vala-level power herself?

And Ancalagon? He drove back the host of the Valar, led by Eonwe, the warrior Maia, and composed of thousands upon thousands of High Elves - yet Earendil slew him in single combat. Is the Silmaril supposed to have helped Earendil, or is he just -that- powerful?

Well, about Shelob and Ungoliant. Here is a recent thread about the spiders. The question that is asked here is- Is Ungoliant of the Ainur?
But, from her actions, we do know that she was very powerful- after all, she destroyed Yavanna's greatest work.

Ungoliant was clearly powerful, but I believe we must not mistake subcreative power for fighting abilities. Melkor always seemed not to have been a great warrior. Fingolfin caused him quite a big trouble and he appeared not to have shown any resistance to the power of Eönwë and his companions, despite being already weakened.

He had shared his powers with his agents and servants, and relied on them to protect himself and harm other peoples of Middle-earth. And it so happened that, his enemies at last being all defeated, he was unprotected and finally caught.

Note however that Ungoliant had powers that even the most skilled of the Ainur did not have...

Melkor had spent a great deal of his power by the point where Ungoliant attacked him, where as she had grown in might due to the power she had drained from the trees. I beleive it says tha much in the Darkening of Valinor. IMO the Silmarils probably dampened his dark powers as well.