If something is spiniest, it has the most spines compared to other things.
The word spiniest makes people think of things with a lot of sharp points, like a cactus or a porcupine.
Examples:
The cactus is the spiniest plant in the desert.
The porcupine is the spiniest animal in the forest.
She found the spiniest sea urchin on the beach.
That is the spiniest chair I've ever sat on!
The spiniest tree in the world has a million spines!
The spiniest fish in the ocean is the lionfish.