If something is spinniest, it spins more than anything else.
The word spinniest makes people think of things that go around and around very fast.
Examples:
The top is the spinniest toy.
The merry-go-round is the spinniest ride at the park.
She is the spinniest dancer I've ever seen.
The fan is the spinniest thing in the room.
The tornado is the spinniest wind in the world.
The spinniest record in the world spins so fast it makes you dizzy.