5. Ra's Al Ghul
Again, you might not say Ra's is a JLA villain. He's a Batman villain, yes, but have you read Tower of Babel? Tower of Babel is arguably one of the best Justice League stories in recent memory. If you ever wanted to see a story where the Justice League was not only crushed but completely and thoroughly defeated without any effort, AND wanted proof that Batman is the most overly prepared hero ever, read it. As for Ra's himself, no, he isn't a physical threat to the likes of Superman or Wonder Woman. He's a great fighter, but his strength, like Batman, lies in his mind. It would be easy for superpowered heroes to initially underestimate him, before his true genius comes out. What's most interesting, however, is that Ra's genuinely wants the world to be a better place, like the JLA. It's just that his definition of "better" is different. Very different. Like in Batman Begins, this makes Ra's into a nuanced, complicated character, which a JLA film needs. Heroes should be memorable, but what's a good hero without a fascinating villain?