I love bad guys. I'm not saying I'd rather be the villain than the hero. Antagonists add a deep layer to any plot. It's what the main character is struggling against at the very core of the story (besides themselves and obstacles along the way).
What is so different about YA bad guys from Adult bad guys? You know, I really don't know. Perhaps they can be scarier, or more evil, because it's dealing with YA topics. While battling ultimate evil and saving the world, the main character must also fight his inner demons (as Luke Skywalker and Harry Potter do), and, since the bad guy reveals these inner demons, it makes them seem more evil.
Not every book or story has to have the ultimate evil character, some show the story of a character battling the inner demons, or killer unicorns, or something of the sort. But make sure you can point in your manuscript to whatever it is the main character must fight. We like conflict. Give us conflict.
Tell me, who is your favorite bad guy? The more evil and complex, the better.