If you measure evolutionary success by what group produces the most new species at the fastest rate, the mammals and the birds win. One group of fishes stands (so to speak) on the podium as well.
Scientists who studied animal divergence report that crocodiles have had 250M years to diverge and produced only 23 species, while mammals have produced over 5,000 species in less than half that time. The study leader commented that it's a mystery "Why these evolutionary losers are still around."
COMMENT: WHat I take away from this is that evolution is tricky, messy, and complicated. For all we know, some of the "losers" may be around long after we furry creatures have passed from the scene.