A really stretchy superhero created by Jack Cole in 1941 and owned by DC Comics.
His real name is Eel O'Brien, and he used to be a small-time crook until he was shot and doused with acid while his gang was robbing a chemical factory. His life was saved by a reclusive order of monks; when he discovered his powers, he was inspired by the monks' example to turn his life around. He disguised himself (easy to do with an elastic face) to allow himself to operate as a crime-fighting superhero while using his old identity as a crook so he could keep tabs on the underworld.
Plas can stretch a long ways, and he can reshape himself any way he wants. He's also very resistant to injury and may be entirely immortal. He's generally depicted as a constant comedian and prankster, though his original characterization by Jack Cole had him as a fairly straight-arrow crimefighter -- he didn't need to make jokes when his entire body was a joke. (Besides, Cole's cartooning made almost everyone else look bizarre and hilarious -- Plastic Man was very nearly the only sane man in his world.)
He's been knocking around DC's continuity since World War II, and he tends to attract writers and artists who appreciate his sense of humor and the fun of great cartooning. He's been drawn and written by creators like Phil Foglio and Kyle Baker (Baker's is one of the best depictions of the character), and he served in the Justice League of America while the "JLA" comics were written by Grant Morrison, Mark Waid, and Joe Kelly. Plastic Man's professional fan club includes Baker, Morrison, Art Spiegelman, and Frank Miller, who considers Plas to be possibly the most powerful character in the DC Universe.
Plastic Man's most frequent sidekick is Woozy Winks, a bumbling nitwit who originally supplied much of the comedy in the stories. Woozy rarely appears in modern-day stories, and when he does, it's usually as a mere cameo.
Plas has appeared in a number of animated cartoons, most notably in "The Plastic Man Comedy/Adventure Show" in 1979, "Batman: The Brave and the Bold" in 2008, the DC Nation Shorts in 2011, and "Justice League Action" in 2016.
Plas is funnier than Mr. Fantastic, and his name is easier to deal with than the Elongated Man, so I will arbitrarily declare him the best stretchy superhero in the world.
Cole was originally going to call the character "India Rubber Man." Luckily, he was persuaded to go with another name.