Zemus isn't the only one though, and Necron is a brief "FF-throwback" at the end.
Most FF main-villains remain unseen until the end of the game, but they are directly controlling all the events that take place throughout the course of the game. Chaos, Ultimecia, Xande and Sephiroth also fall under this category, I would put Yu Yevon in there also, as he is the actual villain, the rest are to tie up the loose ends of the story. To me that's just the style FF assumes in many cases, so I wouldn't call Zemus the worst for that reason.