The Joker?

[Murrocko]

What is it about the Joker that makes him one of Batman's greatests foes. How does he get the upper hand on Batman every now and then, what makes him so god damn dangerous?

[Harry]

The element of unpredictability I guess. Batman's methods are based on forethought and planning which is difficult when the person you're planning around is totally insane.

[Arvandor]

As I say every time this question is asked (which is quite frequently)...

The reason Joker is number one is NOT because he is the most dangerous. It is because he is the most EVIL.

All other supervillains have a criminal's code, an ethical line that they will not cross.
But not the Joker.

[1HELLBOY]

The Joker's unpredictability is, some of the time, predictable. So that rules that out.

Let's take a look at Killing Joke. He shot Barbara Gordon through the stomach. Crippling her. But before he left, he undressed her and took photos of her naked. Kidnapped Jim Gordon. Stripped him naked, tortured him to the brink of insanity(Gordon was stronger than that, which I'm damn glad for).

Now let's take a look at A Death in the Family. He beat a child to death with a crowbar, and then blew him up.

He shot and killed Jim Gordon's wife. With barely a hint of remorse. And yes, there WAS remorse there. He didn't WANT to kill her. He did it anyway becuase she got in his way.

He shot Bruce Wayne's "friend" Tommy Elliot. Seemingly, just for the Hell of it.

Why is the Joker Batman's greatest foe? Becuase he doesn't attack Batman head-on and with his fists. Joker's attacks are of the mind. He attacks everyone around and involved with Batman, in hopes of driving Batman insane, and THEN Joker will kill him. IF he feels like it. Mr. Freeze, Bane, Killer Croc, Man-Bat, they all attack with physical force. Joker attacks Batman's heart and head, emotionally and mentally.

THAT'S why Joker is Batman's greatest foe.