Answer:
There are a few different kinds:
Drones (Buggers)
Grunts
Jackals
Skirmishers
Brutes
Elites
Hunters On toughness scales:
Drones (Buggers) 1/10
Grunts 1/10
Jackals 3/10
Skirmishers 4/10
Brutes 6/10
Elites 10/10
Hunters 8/10
If you play Halo: Reach, here is a good way to get your CR up.
Go to Firefight
Set all loadouts to:
Primary: Rocket Launcher
Seconary: Gravity Hammer
Set it to bottomless clip
Set map to Courtyard
Set all enemies to Elite Generals
Have fun killing Elites :)