I play Cheetah and Firestorm (mainly) so I'll do this for them.
Least favorite for Cheetah is easily Batman, I feel like he has a lot of great tools against her.
Favorite for Cheetah is Black Canary. It's not the easiert match up, but to me, it's fun when 2 rush down characters go at it.
Least favorite for Firestorm is Catwoman, she's so mobile it's hard to get anything started and with Firestorms lack of a really good anti air, she basically gets in for free. Or maybe I'm just not that good at the matchup yet, which is probably the case.
Favorite for Firestorm is Scarecrow. I feel as if Firestorm shuts down a lot of Scarecrow's offense. You just have to get him off you and it's pretty simple from there.