Black Manta is without a doubt Aquamans most recognizable foe, so hes obviously the first evil candidate that comes to mind for the Aquaman movie.
Black Manta has cemented himself as Aquamans arch-nemesis, so using him as the movies main antagonist would be a great way to swing strong.
Hes one of the most visually distinctive bad guys in the DC Universe, and that look is just begging to be brought into live action.
However, recently director James Wan talked about how interesting he found Aquamans cast of characters, but wouldnt reveal which bad guy hell use in the movie.