There have been many post about the joker being robin but the question is, which of the 3 robin's (Dick Grayson, Tim Drake, Jason Todd) ?
Well there could only be one and that would be none the less than Jason Todd. If you Played ARKHAM KNIGHT yet (Spoiler Alert) you'll get a scene where The Joker is torturing Jason ( this is when you are trying to save the the 3 infected joker's ) after 6 months of Jason being held hostage, the Joker then says "Your my sidekick now". This could be a hint towards The Joker training Jason to become the next Joker.
Also if you see Joker's ( Jared Leto's ) Left check you'll see a the letter J in form of a tattoo
In ARKHAM KNIGHT boss battle with the arkham knight aka Jason Todd, you'll also see the letter J on his left check
For those DC fans out there, they probably know that before the joker was the joker, he had the name of "Red Hood"
hinting that the Joker (maybe Jason Todd) wanted to take on the name of "Red Hood" again, or something along the lines.
But although this all might make sense of Jason Todd actually being the Joker, there is one part that doesn't match or ever will, if Jason is the Joker then who is the Joker that beat up Robin (Jason Todd) in that old warehouse? We're there two Joker's, or is there only one? Who was or is the first Joker?