As far as I'm concerned there are four compelling candidates for this part. In no particular order: Toby Jones, Paul Giamatti, Steve Carrell and Timothy Spall.
Out of those four fine actors, I'd personally give the edge to Steve Carrell. Unlike Jones and Giamatti, he has not yet played a comic-book character and he arguably has the highest profile and most star charisma out of them all (and although the classic Penguin may be a creepy and peculiar-looking criminal, as a powerful mob boss, club owner and even celebrity among the Gotham elite, he should possess as certain degree of charisma). Plus, Carrell demonstrated his dramatic chops with Foxcatcher where he played a creepy aristocrat, somewhat akin to The Penguin.
But any of those four guys would be great. Although it's a shame that Philip Seymour Hoffman, the best Penguin that never was, is no longer alive.