D4 is latest and greatest.

D3.5 is the fastest but is not ROAR legal. Killshot came after the D3.5 and released before the D4.

Big blue is not ROAR legal since the stator wire is thicker (which is why the D3.5 was deemed illegal). Maxzilla is just a term for a motor coming equipped with 12.5mm turquoise torque rotor. You want to buy it with this rotor unless you are running 1/12 or F1 where you don't need the torque as much.