Joker is probably 3rd or 4th in terms of being any real threat to Batman. His whole schtick is being unpredictable; one day going through an elaborate scheme to throw a pie in the Dark Knight's face, or just walking into the Gordon home to shoot Barbra. Ras Al Ghoul, Bane, and Mister Freeze are the sorts who are threats on all levels to Bats. Joker only has the fandom he does because people love his utter sociopathy for some reason, or because he was played by handsome deadguy actor (who's interpretation was sub-par to be completely honest. His version of The Joker was little more than the kid who set cats on fire and robbed the convenience store in highschool, and never grew out of it. Mark Hamil and Jack Nicholson were much better, because they actually capture that the Joker really is that corny in his sociopathic behavior).