Rubber flooring is the safest and most durable material to use under a swingset or other playground equipment. Playground flooring needs to be shock-absorbent to protect a child from injury if they fall, and rubber is about as shock-absorbent and durable as it gets when it comes to flooring. For a swingset specifically, you may want to consider rubber mulch and a playground mat to prevent any worn down areas in the mulch.