That is a hard question, because there is too much to choose from!!! Well, for starters I will have to say that I have worked for a while with copepods and they are really great! I used to work with Acartia which is a pelagic species (lives in the water column), but bentHic species (that live in the bottom) such as Tisbe are really coll also! (Google them for pictures!) Tisbe will carry their eggs by the side of the body everywhere and then when they hatch the little ones moult (change body and shape) several times until becoming adults 🙂 on Acartia they will lay their eggs, sometimes up to 50 per female and they can survive in extreme conditions (they are called resistance eggs)…some were put in the fridge and 10 years after they hached into little babies with no problem! How cool is that? 🙂
I think it would have to be the microbes that create Stromatolites. These are only found in Western Australia and form layers of sediment that form rock-like structures. The layers are formed by a “biofilm” of bacteria-like organisms. These are most similar to the first life ever recorded on Earth, going back ~3.5 billion years.