The reporter also asked Thomas Edison what was the hardest invention to develop. He answered quickly again -- the electric light. He said that it was the most difficult and the most important.
Must I add 'the most important invention' or 'one' at the and of the sentence or is it ok to just say 'the most important''?
It's okay without it. If you want to add something, use "one" and don't repeat "invention."
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.