to my opinion, you have the energy sword, and the shotgun, and the assault rifle. but it defends on the difficulty or what weapon you choose.
Agreed the flood are easiest to take down with a shotgun or energy sword but the assault rifle and the battle rifle are the best to take down flood at a medium range.
well not exactly the enegy sword as it is a fast accurate weapons. for the flood you'll need as said above along with a flamethrower and a gravity hammer as it is a wide range weapon