Morena Baccarin is something of a staple in Nerdom (I'm not calling all Tolkien fans nerds... pff, nerds...) popping up in everything from Firefly, Justice League cartoons, Stargate, DC-TV, and most recently the Deadpool movie. The always beautiful Ms. Baccarin can bring just as much of a dark and sexy nature to Ungoliant as her fellow nominees. But Morena Baccarin brings something else: that spider-like physicality and gaze that makes you not quite sure whether you want to fight her off or run in terror or fall in love.
Morena also has a lot of experience in playing both heroines and villainesses. I think her comfort in either type of role would help bring an ambiguity to Ungoliant and her intentions. Think it would be fun to toe-the-web so to speak. She could turn a seemingly one-dimensional, evil spider into a more rounded, interesting character.
While it would be easy to do the Mortician Addams thing with the long dark hair and pale pale skin, I think we should cast the pre-giant spider Ungoliant with somewhat more subtlety. When she turns into a monstrous all-devouring spider, it would almost be a shame if our viewer were like "Yeah, saw it coming."
I would like to nominate Ruth Negga. She is stunningly attractive, has a beautiful voice in a lower (and some might say sexier) register. She in an accomplished actress who has performed for movies, tv, and stage.
I first came across her as the cold and manipulative Raina in Agents of S.H.E.I.L.D. In my opinion that character outshone all of the other Bad Guys that she was working for; and I can easily picture this actress playing someone who is capable of creeping out even the normally very self-assured Melkor.
Bonus: she is used to acting in crazy makeup and prosthetics