Against Grunts, you can use everything. Just watch out for the suicide ones.
Against Jackals, same thing, but I recommend using headshot weapons.
Against Brutes, use a Plasma Pistol’s EMP to disable his armor. After that, most weapons are viable to kill them. Weapons that are good as well to disable the armor are Spikers, Plasma Rifles, Assault Rifles, Needlers (Supercombine will outright kill them), etc.
Against Hunters I recommend using a Shotgun, Energy Sword, Gravity Hammer, Rocket, Fuel Rod and Splaser. If you’re using Shotty, Sword and Hammer, get up close to them, evade their melee and hit their back. A Sword is EXTREMELY good in the Hunter’s back and can kill them in 1 swing. The Hammer may make things easier and you may not need to wait for his attack animation at all. A couple of swings can kill them pretty quick. If you’re going Shotty, when you’re attacking his weakpoint, shot and melee. If they get stunned, repeat, then evade.
Against Drones, only hitscan weapons are viabe (hitscan means that your shots have no travel time between you firing your gun and the shot hitting it’s location; in other words, instant. Assault Rifle, BR, Sniper, etc), because they’re too fast against Projectile weapons (Weapons whose shots need to travel to reach their destination. Plasma Rifle, etc.)
Against the Human, Elite and Brute forms, almost all weapons are viable, even your melee, but I recommend using Plasma Rifles, BR, Shotgun, Energy Sword, Spiker, Brute Shot, SMG, and Carbine and Gravity Hammer.
Against Pure Forms, the same thing, but you must be more cautious. And melee is only useful against Stalker forms. The Tanks are too resilient against your fists and getting close to one may only mean a free ticket to Death. And Snipers are usually on walls and ceilings, out of your melee reach; the melee is strong against them, but not viable.
However, the Energy Sword WILL kill every Flood form in just one swing. Not only are they insta-kill, but they also waste very little energy against them, no to mention that Energy Swords are common, especially in Cortana. I recommend you to keep a Sword at all times on Flood levels.
Hope this helps. Now Finish the Fight once again.