Despite his distinct appearance and memorable gimmicks, the Joker has actually proven to be a fairly effective master of disguise over the years. He's even been able to slip right under the nose of Batman, the world's greatest detective.
Here are the best disguises the Joker has used during his criminal escapades.
Golden Age Disguises
The original Golden Age version of the Joker has a habit of using other disguises and false names to hide his identity and intentions. These include hiding in plain sight as A. Rekoj, a music store operator who allows his business to be used as a front for criminal ventures. After hiring men to steal for him, he resumes his identity as the Joker and steals the entire prize from them, leaving them confused as to how the Joker knew of the robbery in the first place.
In another criminal escapade, he poses as a visiting nobleman in a bid to make off with a great deal of aid provided by the Gotham populace.
These disguises often give the Joker a brief advantage in his criminal attempts, but each is in turn foiled by Batman and Robin. It was also during this period that the concept of the Red Hood was introduced, revealing Joker had used a separate, criminal identity before the fateful night he fell into the acid at Ace Chemicals and was transformed into the Joker.
Oberon Saxton
After seemingly dying during the events of Batman R.I.P, the Joker briefly goes into hiding. Disguising himself as the famed British journalist Oberon Sexton (who Joker had buried alive with his dead wife), Joker indulges in Sexton's masked persona as a gravedigger to hide his appearance from people such as the new Batman. He briefly works with Dick Grayson while he serves as Batman and seems to throw his hand into an investigation into the Black Glove -- despite misgivings from Robin, who notes something is off.
At the time, Joker is also secretly operating as the "Domino Killer," a mysterious figure targeting the members of the Black Glove and assassinating them with otherwise mundane tactics. Realizing the clues left by the murders point towards a series of jokes, Grayson eventually confronts Sexton -- who gleefully reveals his true identity. Joker is quickly taken into custody, with the rest of the Gotham Police Department shocked they'd fallen for his trick.
Re-donning the Sexton costume, Joker later confronts Thomas Hurt head-on and tricks him into breaking his neck after slipping on a banana peel.
Eric Border
After failing to fully destroy the Bat-Family in "Death of the Family," the Joker goes into hiding. This allows Batman to become more distracted and concerned with other threats, giving Joker plenty of time to develop a new persona. He re-emerges as Eric Border, a new orderly at Arkham Asylum. Hiding his permanent smile with muscle relaxants and his pale skin with makeup, Eric establishes himself as a kindly and supportive figure in Arkham.
He even becomes something of an ally to Batman, lulling the Dark Knight into a false sense of security from which he is able to operate in secret and manipulate Gotham into place for his grandest scheme ever. He quietly set traps across the city and even terrifies patients into believing him to be some special form of supernatural monster.
Joker finally reveals himself at the beginning of the "Endgame" story arc. He lures Batman into the closed ruins of the former Arkham Asylum and traps him there, giving Joker time to move to the next part of his plan.