If something is spikiest, it has the most spikes or sharp points compared to other things.
The word spikiest makes people think of sharp things and maybe a little bit of danger.
Examples:
The cactus is the spikiest plant in the garden.
Out of all the animals, the hedgehog is the spikiest.
The spikiest part of the fence is at the top.
I don't want to touch the spikiest part of the toy!
The spikiest mountain is very dangerous to climb.
The spikiest monster in the story scared me the most.