The enemies in the Halo series are Grunts, Jackals, Drones, Elites, Brutes, Hunters, Prophets, and the Flood. All of these are incredibly satisfying to kill.
The Unggoy hail from Balaho, the fourth planet in the Tala system. their world has an atmosphere of carbon dioxide, methane, and helium. They cannot breathe oxygen so they wear breathing apparati in order to fight. A Grunt without it's mask is UGLY!!! They have an impressive capacity for languages but every language they speak sounds like it's being murdered by a chipmunk on cocaine. They are physically unimposing so they need to attack in groups and usually do so with a larger species leading the pack. They contribute to the game as cannon fodder, combining the rush of shooting small animals with the fun of shooting foreigners! They're also incredibly stupid, if stuck with a grenade, they'll run towards their fight group and thus injure/kill everybody. Also, in Halo 3, suicide grunts were introduced, confirming that the Grunt homeworld is really just the Middle East.
These guys are a substantial step up from the Grunts. They are the Covenant's Mercenaries, functioning as snipers, shock troops, and legitimate soldiers. Not only are they physically imposing, they have excellent senses and are well equipped with an energy shield that blocks most weapons. They also have no qualms about eating fellow Covenant members. They are vicious little fuckers, don't mess with them.
These guys are annoying. They are the only enemies that can fly and thus are hard to get a good bead on. One good thing about them though, they have low health, a few shots and they are dead. These guys attack in swarms, like 30 at once and since their favorite weapons are the needler and the plasma pistol, get ready to die, A LOT. You will be dodging pink needles and green energy blasts by the sackload.
These guys were the Super-Soldiers of the Covenant for many millenia and one of the harder villains in the first and second games. They are physically challenging, incredibly smart and versatile, and can use just about any weapon. They lead phalanxs of grunts into battle and also pose a boss-like threat in many points during the game, with the Ultra elites clad in trong gold armor and weilding an energy sword. They also pilot most of the vehicles in the first two games. One good thing about them, they are BIG enemies, which means they are easy to stick with plasma grenades and when they are stuck they flail around hilariously. In Halo 2, you get to play as an elite, the Arbiter, who is a BAMF. When he's not boning sweet elite bitches, he's slaughtering hoardes of flood with his energy sword and then coming home to decapitate his former leader and spartan-kick him over a ledge into hell. In Halo 3, elites and humans fight together. This is pretty cool!
These guys are apes on steroids. They live in a warrior society where the guy who has killed the most, sexed the most, and generally won the most is in charge. These guys are tough on normal and nigh impossible on mythic. They have very sturdy armor, tough hides, excellent weapon-handling skills, and the ability to explode into super-roid-rage when you hurt them enough. They are a threat to everything around them and their weapons are all incredibly powerful. the best example of this is their signature weapon, the gravity hammer. this is a giant stick made of metal, with a huge bludgeoning head on it that can manipulate gravity. it can force you down into the dust or throw you against the wall, and if it hits you, say goodbye to your bones.
These guys are tanks. They will kill you then sodomize your corpse. They are colonies of worms combined into a giant slug and then integrated into a mechsuit. Needless to say, Japan would have a field day with these guys in their porn vids. Setting that aside, these guys always attack in pairs and have giant fuel rod launchers attached to their arms. ATTACHED TO THEIR ARMS!!! These are actually rare forms of the Lekgolo, sent to the Covenant as Ultra Troops.
These guys are the spiritual leaders of the Covenant. They are manipulative assholes. If a mormon tries to talk to you about religion, you just shut the door. If a prophet tries to talk to you about religion, they will probably annihilate your solar system afterwards. They make that bitch who got everyone in grade school to hate you look like a saint. They aren't really an enemy as you only "fight" one in the whole trilogy and all you really do to him is punch him in the face until his head implodes, satisfying, but hardly a fight.
These guys are mutant zombie things. They infect the bodies of organisms and transform them into walking nightmares. You fight a few forms in the game, the pipsqueak infection form which is only threatening in extremely large groups or on mythic; the 'splodey bag which explodes when it gets near you, BE CAREFUL!!!; the incredibly ugly and frightening mutated human form which still knows how to use a gun (as if fighting zombies wasn't enough, now they can shoot at you too); the mutated elite form which is super strong, and super fast; the brute form, which just charges at you and has tons of health; and the pure form which can mutate into the spike launcher, the spider, and the ultra-tank, all of which take several magazines to kill.