I think in the entire Covenent army, the two most irritating enemies are the Jackal and the two Elite versions. The opne carrying the Plasma Rifle and the other that carries the laser blade.
Jackals hide behind the shield and fire. Sometimes, if you are not careful, you may be hit by the overcharged blast from their Plasma Pistol and your shields go to zero and warning alarms start buzzing from your speakers. Then I have to retreat back to the last corner and wait for my shields to come back up.
The Elites are equipped with shields as well. So, they take a longer time to kill and they are very agile and like to roll a lot so that you cannot hit them. There are also the cloaked versions with energy blades that can kill you in one swipe.