The Rocket Launcher. It kills a Hunter in one shot. With the Fuel Rod launcher, two shots. With a Scorpion or a Wraith, one-shot kill.
Sniper rifles can also kill in one shot, but only when shot in the exposed flesh in their backs. Pistols also are very efficient in the first Halo game.
Spartan Lasers are the most powerful infantry weapon in the entire Halo series, with the exception of the Halo 2 Scarab Gun. They can take out a Hunter in a single blast, unless their shield blocks the shot. Even then, they can still die from the splash damage.