Plastic Man is the most powerful of the stretchy heroes, but Elastigirl is fucking bad ass.
Plastic Man is (currently) a goofball (he was much cooler in the golden age, frankly), and Reed Richards tends to have his head in the clouds. Elastigirl is pragmatic, efficient, and ruthless. She also has an indestructible costume covering almost her entire body.
Folks are still forgetting one, Offspring, Plastic Man’s son. He has Plastic Man’s powers, but squared. He can not only stretch and change shape, he can change his density and color. Batman considered Plas as one of the most powerful beings on the planet (He survived 3,000 years broken down into little pieces until the JLA managed to find and put the pieces together again.) and Offspring is even more powerful.
Actually, at first, she could only grow and shrink (and for some reason, her clothes grew and shrunk with her), but kept the same proportions. In the origin story, she had no control over herself and grew too big and wrecked the set.
But stretching superheros are very common in comics.
Plastic Man is tops, but only because most people are mistaken about what his abilities are. Not only is he pretty much indestructible and immortal, but(as The Batman figured out) his ability has nothing to do with stretching-his ability is transforming. When he takes on the shape of a lamp post, he actually lights up. When he takes on the shape of a hot air balloon, he floats. If this dude existed in the Marvel universe, he’d be an Omega-level mutant.
In the Marvel Universe, Mr. Fantastic is inerringly right. Even if Plas somehow managed to get an upper hand, his adoring and obedient family would come through for Reed.
In the DCU, the heroes usually manage to beat fascist villains. So Plas wins.