What existing life on Earth is best suited for the Martian environment? By best suited, I mean it could theoretically be placed on Mars and easily adapt or thrive within the existing environment the planet has to offer.
I know from my own research that extremophiles are potential candidates. There is a discussion about Could any life from Earth survive in Enceladus' oceans? which is relevant to my question, however this focuses on bacteria found in water only. There is a question on Is it possible to grow plants on Mars? which is related, however not exactly the same, and from the information that the answer contains leads me to believe that plants would not be good candidates for my question. There is also a discussion which relates to the search for life on Mars and how we could contaminate it with human boots on the ground.