They are all within a dynamic equilibrium. Snake beat Toads, but Slugs beat Snakes, however Toads beat Slugs, making it equal, and very much like Rock-Paper-Scissors. Each one has their own utility that makes them powerful and unique. However, from what we've seen, toads have the most versatility and damaging potential because of various applications from Ma and Pa and their nearly non-counterable genjutsu (Frog Song).