Which organim's DNA will differ the most from the leopard? Why?

The "lancelet" DNA will differ the most from the leopard.

Explanation:

The lancelets are also called cephalochordates (Greek: kephale, "head") because the noto chord extends from near the tip of the tail to well into the anterior of the body. Because they do not have the brain case, or cranium, of a vertebrate, lancelets are often called acraniates. They were traditionally held as the closest living relatives of vertebrates and have long occupied a key phylogenetic position in our discussions on the origin of vertebrates.