My least favorite is Pulse grenade, it takes like 4-5 seconds for another bang and enemies retreated from it.
The Focus Rifle. Never got the hang of it, and perhaps it's better than I realize, but it always came off to me as the odd result of a Beam Rifle and a Sentinel Beam having an underpowered baby.
Plasma Repeater because it is simply unusable. It's the most unaccurate when in all of halo, does little damage to compensate for it, and overheats way too fast. The suppressor from Halo 4 is a close second except it is more accurate and can be used to kill somewhat effectively in super close quarters.
Railgun, Saw, H4 Binary Rifle, Surpressor, H4 Boltshot, DMR, H3 Pistol, H4 Boltshot, Hydra Launcher, The weird Promethean Grenade from H4, the grenades in H5, H2 BR, every plasma rifle after Halo 2