The Joker's intelligence is maybe the most underplayed aspect of the clown's characterisation. While he's not the top of the list of IQ-wielding villains, it's only through the Joker's meticulous planning and eye for fine detail that any of his dastardly plans end up working out.
Perhaps more importantly, the Joker's surprising level of emotional intelligence means he's able to manipulate both friend and foe supernaturally well. Combine this with a penchant for chemistry, and it's easy to see that the main reason the Clown Prince of Crime is such a threat is down to his seemingly endless capacity to be cunning.
And plenty of storylines make this readily apparent, with the Joker conniving his way into (or out of) all sorts of amoral acts. Although his schemes rarely go entirely to plan, it's surprising just how often the jester outsmarts people - especially when he knows doing so would create a little chaos.
Be it through poison, magic, or just a humble explosion, the Joker is always willing to do that extra mile to get the last laugh - even if it involves snapping his own spine to do so.