Probably:
-Catwoman
-Bane
-Deadshot
-Talon
Anyone else?
Probably:
-Catwoman
-Bane
-Deadshot
-Talon
Anyone else?
Clayface, Man-Bat, and Croc who are already written as allies
Two-Face maybe
I wanna say Freeze but that will be boring since all he does is stay in his lab
Those who received a comic book with their name on it.
What do you mean "easily written as protagonists"? If it means "in how many kinds of stories that character can be written" and "doing things that dont turn off the readers" then morally grey characters like Catwoman. If its about making the readers understand their viewpoint (hero of their own stories narrative), understand the pain & tragedies that shape them into who they are and motives for their immoral acts and even feel sympathetic to them then Mr.Freeze & Ivy. I dont count Harley because its her popularity that pushes her to be written into more of her own character when the point of her character was originally a sidekick that was willing to commit any atrocity to please Mr.Jay. It was a completely selfish reason to commit crimes, therefore making her not really sympathetic.
I think Riddler is actually challenging as a protagonist because you'd have to have a writer who can at least pretend to be as smart as him. And being able to pull that off month after month would not be easy. Someone like Batman who's as smart if not smarter at least has a lot of other things going on, or stories where he doesn't use his intellect. But Ed? His whole mind works in riddles - I'd love a well-written mini seeing what it's like to be him, but an ongoing might be difficult to pull off, imo.
The character himself may be difficult to write, but setting him up as a protagonist isn’t all that hard, as he’s all about matching wits, and doing it against other villains wouldn’t bother him.
TBC I’m talking about two different things here. Powers/abilities and motivation.
As an example, The Flash would be hard (for me) write because I’d have trouble challenging a speedster and portraying speed in a suitably technical manner. But he’d be easy to write in terms of actual motivation because he’s a good person.
Riddler’s motivation is proving he’s the smartest guy in the room. Usually that means outwitting Batman, but not exclusively.