Always amused when writers create a character like this, and seriously try to convince us that a mean with the superpower of elasticity will use it to, uh, turn his hand into a hammer for some reason.
Plastic Man is, unironically, one of the most powerful heroes out there. He's basically invincible, being able to be reduced to the atomic level and still able to regenerate eventually. He can increase his size, strength, mass and density at will, his brain is plastic and cannot be read. Even Batman has only found a way to slow him down, not stop him entirely.
His weakness is basically that he's not the smartest and he rarely takes situations seriously.