I am extremely interested in learning more about the Mgalekgolo, Sangheili, San 'Shyuum, and Huragok.
Mgalekgolo: The Mgalekgolo recite war poetry, meditate, and form extremely close bonds with each other. Combine that with the fact they can assimilate into thousands of forms, they are amazingly interesting. They also make up some of the Covenant's most power forces, such as Hunters, Harvesters, and Scarabs. They are adept at working with technology too.
Sangheili: A proud, warrior people, the Elites are possibly the closest thing to humans in the Covenant in terms of appearance, society, technological advancement, and loyalty. They have lead the Covenant forces into many successful battles and are the most powerful race within the Covenant.
San 'Shyuum: Genetically unsound, I feel nothing but pity for this homeless race. They were the driving force behind Humanity's near-extinction, but they have had their own struggles. They can barely function without their anti-gravity belts and chairs, and they must deal with a government that only favors those who will betray everyone.
Huragok: One of the Forerunner's creations, Engineers are mysterious. They can reproduce in hours, are partially mechanical, have microscopic cilia on their tentacles, and can sense the state of technology. It is them we have to thank for most of the Forerunner tech used by the humans. With an adequate force of thousands of Huragok (and a good translator) Humans could be on par with Forerunners in a matter of years. A few Forerunner A.I.s wouldn't hurt either.