To every hero there is a villain, and if we're going to name all the villains the Titans met in the show, we have to start with the one that means the most to the show: Slade.
Ironically, Slade was an intentional plant that was not discovered by those who didn't know the comic. Slade is Slade Wilson, aka Deathstroke, a master warrior and strategic mastermind who seemed to always be ahead of the Titans from the word "Go!".
But what made Slade so creepy, so evil was his obsession with Robin, he wanted him to be an apprentice to him, no matter the cost, even making it so that if Robin didn't do so, he would kill his friends with the push of a button. No matter what the Titans did, Slade always kept coming back, even after he died at the hands of Terra.
Slade will go down as one of the greatest villains portrayed in the animated series. Oh, and it didn't hurt that he was voiced by the one and only Ron Perlman.