Originally Oroku Saki, the Shredder leads the robotic Foot Clan against the heroic reptiles, stealing super-weapons and kidnapping April O'Niel. Shredder is the prime example of villain entropy - the very beginning of the series, Shredder was a competent villain with realistic goals, but quickly devolved into plans that made no sense with zero chance of accomplishment and being defeated by the least amount of effort by the Turtles. By the later seasons, I seriously felt bad for Shredder.
Another one of the most recognizable voices in cartoons, Shredder was voiced by James Avery, who almost anyone knew as Uncle Phil on the Fresh Prince of Bel Air. I can't even think of another cartoon character, villain or not, voiced by him. After looking at his filmography, all I could recognize was War Machine in the Iron Man and Spider-man cartoons. I honestly wonder how TMNT picked him up for this role, but it was a great bit of casting.