Riddler. Aside for my love of Frank Gorshin on the tv show and his character on the animated series, Riddler is probably batman's most fascinating villain because he's just as insane as the rest of them but he doesn't believe it. He believes he's so above the rest of society, the rest of the rouges gallery, and even above Batman due to his intellect but his greatest strength is his greatest weakness. He's compelled to leave riddles because he wants the attention for his genius but that attention is what gets him caught every single time. I know there's a story written by Paul Dini i believe, where Riddler says he won't leave a riddle for batman and he almost pulls it off, almost pulls off his plan but he cannot resist the urge to leave a riddle which ultimately leads to his undoing as it normally does and he checks himself into Arkham because he realizes maybe he truly is crazy