system
Given the following post, respond as if you're commenting on Reddit:
human
What two creatures are most closely related genetically, but look the most different physically?
gpt
This is a little bit of a cheeky approach to your question: the thing most genetically similar to any organism is itself (or its clone/identical twin). A caterpillar is genetically identical to the butterfly it will become, but is vastly different physically. The same goes for any animal with metamorphosis: frogs, jellyfish, crabs, flies, starfish, etc.

Another, more serious answer is tunicates (also known as sea squirts) are the most closely related creatures to vertebrates, but have almost no recognizable "vertebrate" characteristics. They barely have a nervous system, let alone a face.