The strongest flood form, though it is not normally fightable, would have to be the Juggernaut. Its long tentacles would be good for long range and they're still powerful enough to kill you in one hit.
What about the Thraser from Halo Wars? On Normal and up, they can take a direct Scorpion gun round.
The juggernaut form is over 5 metres tall and has the power of a tank while the thrasher form is only two metres tall