The most rubbery or stretchy.
This word makes people think of things that can stretch a lot, like rubber bands or bouncy balls.
Examples:
The rubberiest band can stretch the most.
This ball is the rubberiest one I have.
She has the rubberiest toy in the world.
The rubberiest tree in the forest can stretch to the sky!
The rubberiest material in the world can stretch around the whole Earth.
The rubberiest thing in the world is a rubber band.