Easily one of the more clever villains in the show, Mad Mod is a British fellow who thinks that the Teen Titans deserve to be taught a literal lesson. And in his first appearance he uses a "school" to try and teach them said lessons...by literally turning their brains to mush. Yeah, not fun.
Mod is brilliant, and was able to run circles around the Titans multiple times, and even outsmart them more often than not. Only through the tiniest of fractions did he lose to them multiple times, making him one of the most dangerous villains they've ever faced.
Oh, and he was voiced by Malcolm McDowell, that's instant credibility.