Riddle me this, Riddle me that.

'Batman Will Battle The Riddler'.

Batman will take on villain The Riddler in the next installment of the superhero franchise, according to the Caped Crusader's butler Sir Michael Caine. The British star of both Batman Begins and The Dark Knight has let it slip that his movie boss' next foe will "probably be The Riddler" - a role made famous on screen by Jim Carrey in 1995's Batman Forever and on TV by Frank Gorshin, who died in 2005. Johnny Depp and Eddie Murphy are both in the frame for the role, according to Hollywood gossips. Caine has no idea who will be cast, but is hoping for a performance to rival Heath Ledger's Oscar-winning portrayal of The Joker in The Dark Knight. Caine tells People.com, "I thought it (The Joker character) was going to be a tough job for anyone who played the role after Jack Nicholson (in 1989's Batman). Now Heath is the Joker."

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Now I don't really like Jim Carrey but, as I have said before, I gotta give him his props. He did a great Riddler. Stellar! Top-Notch. He carried the movie totally, they didn't even need 2-face there (and really shouldn't have either, even Tommy lee Jones was over-shadowed and I LIKE him).
Eddie Murphy as the Riddler? No. Not even. He is too old and "over" and I dont think he could be maniacal enough.
Johnny Depp? Yeah I could see that. He could take the character into another quirky direction/dimension. It would be different but good.

Who would I choose? Glad you asked. I think I would cast an unknown actor. Give someone their big shot because they would give you everything they had for the role. Even Ledger, while good as the Joker, still didn't give 100% of himself for the role because he had already tasted stardom.
Put someone in who this is their big one-time make it or break it shot. They will shit roses for you.