Powerfulest is definitely a less common form of superlative compared to the more commonly used “most powerful” ,but, as shown in Google Books the term is used.
From: The Portable Walt Whitman
By Walt Whitman
Over those politicians and great and little rings, and over all their insolence and wiles, and over the powerfulest parties, looms a power, too sluggish maybe,
From Stalker's Creek by Will Cade
He ' s the powerfulest man in town , and the closest thing to a rich one . By now I reckon he really is rich . Hey , want me to ask him ...