Man that's tough. For just outright collectibles...I'd probably have to say Batman. He probably has the most figures made, he's got a statue line (Black & White) that has been coming out for nearly 20 years. So many different suit variations that they make toys for. Probably Batman.
Spidey, Supes, Deadpool...they are definitely super popular and have a wealth of product representing them...I think Batman runs away with it.