This article suggests we might be able to terraform Mars with some very tough moss.
The scientists subjected whole S. caninervis plants to conditions typically found on Mars: high doses of gamma radiation, low oxygen, extreme cold and drought. They report that the plants could withstand combinations of these conditions, even losing over 98% of their water content and still bouncing back within seconds —”drying without dying” is the term that was used. Perhaps even more astounding is the plant’s ability to recover and grow new branches after being stored in a freezer at −80 degrees Celsius (-112 degrees Fahrenheit) for five years or in liquid nitrogen (-195.8 degrees C; -320.44 degrees F) for one month.
Space.com
Previously, my money was on fungi, but some combination of moss and tardigrades might be able to evolve into intelligent life in a few billion years. Now that it looks like humanity is probably going to destroy itself on Earth long before it develops viable space colonies, it’s more important than ever that we broadcast our devil spawn to other planetary bodies as soon as possible. If life on Earth more generally survives, whatever the cockroaches evolve into will be able to talk to whatever the moss evolves into in a few billion years, and there will still be hope for peace in the solar system for a few billion years before the sun burns out.