Zatanna: For as powerful as Superman is, if Zee says “lateleks slessum post gnikrow” (skeletal mussels stop working), he’s immobilized. If she says “caidrac slessum post gnikrow” (cardiac mussels stop working), well, Superman’s not quite that invincible. I’m pretty sure Zatanna is the most powerful DC character.
Hawkgirl: Hawkman is a tank, Hawkgirl is a tank with a brain. This is a bit trickier because she as a character varies so much, but traditionally she’s the one to stratagise more. Also, according to the animated JL series, she figure out where the batcave was and how to get in. And even if Batman had previously told her, she still kept her secret for two years without him ever knowing. That version aside, she is a good detective with enhanced vision and a birds-eye-view. She’s not on Batman’s level, but she is a good counter. Also, they don’t typically have any emotional ties to manipulate each other; it’s strictly business.
Gentlemen Ghost: He’s good at getting under people’s skin, and then makes it ten times worse when you can’t even punch him. I think he could keep Wonder Woman busy long enough for Zatanna to finish with Superman at the very least.
Poison Ivy: You know how annoying it is when you’re running through the woods and you trip over a knot of wood? Yeah, now imagine if you were trying to run at the speed of light, but every plant along the way keeps wrapping itself around your ankles. And as soon as you’re free of one, there’s a dozen more ready to grab on.
Beast Boy: You really do need another shape-shiftier to take on a shape-shiftier. And while it could be argued that Martian Manhunter has the upper-hand with his ability to read BB’s mind and thus predict what he turns into, one has to actually think before they act for that to apply.
Captain Marvel: Water + lightning = smoked salmon. Also, his childlike innocence has it’s own benefits.
Dexstar: All powerful weapon wielded by an angry cat? I don’t even know where that could go, but I’m betting it’ll be enough to throw any lantern for a loop. Cat’s can be very unpredictable and their brains work very differently from humans; that’s were the true strength of this character lies.