Here are the best weapons for defeating certain enemies:

Grunts- SMG, Battle Rifle, Dual anything.
Jackals- Covenant Sniper Rifle
Elites- Dual Needler, Energy Sword, Sniper Rifle
Brutes- Energy Sword, Sniper Rifle
Prophet dudes- Anything you can use.
Humans- Energy Sword, Rapid Firing guns
Sentinals- Sentinal Beam
Giant Robots- Dual Needler, Sentinal Beam, Rocket Launcher

Flood- Energy Sword = One hit kill