If something is slippiest, it is the most likely to make you slip and fall.
Slippiest things are dangerous and can make people hurt themselves.
Examples:
The ice is the slippiest thing I know.
The slippiest part of the floor is near the door.
The slippiest shoes are the ones with no grip.
The banana peel is the slippiest thing in the kitchen!
The slippiest hill is the one covered in ice.
The slippiest part of the playground is the wet slide.