Youma Boxsy!
Seriously though all the attractive guys are mid-bosses instead of monsters of the week so I can't list them. I guess there's Narcissus from Codename Sailor V, but I'm kinda middling on him. And I probably shouldn't count zombie Umino...
I'm quite fond of Garagara Musume (Going with SM Wiki instead of my usually-preferred Wikimoon, because SM Wiki has pictures).
Thetis is also very pretty, in either form actually. Her red eyed mouthless look is intriguing and cool.
If we're talking just human forms, the vampire girl from the SuperS special was pretty in human form (but not so much as a monster).
Personally, I think Yumemi Yumeno was cuter than Binah, but both were cool.
Also, they aren't beautiful in the sense of attractiveness, and they're kind of borderline for monster-of-fhe-week, but, Nehelennia's illusionary flowers were really cute.