Just curious to see what people think. I’m gonna list one rank per species.
Grunt: Heavy (H2,H3,ODST,HR,H4)
Jackal: Major (H1,H2,H3,ODST,HR,H4)
Skirmisher: Murmillo (HR)
Elite: Honor Guard (H2,Wars)
Brute: Jump Pack Captain (ODST)
Drone: Leader (ODST)
Hunter: Gold (ODST)
Engineer: Support (Wars)
Prophet: Hierarch (H2,H3,ODST)
Whats your favorite?
my favorite is elite councilor or the arbiter.
Either elite commander or jet pack elite.
I’d say Honour Guard, with Councilor coming in a close second for the elites. The brutes would be a Brute Chieftain, and the grunts would be an Ultra.
Other species in the Covenant, I don’t really have a favourite.
The Zealot Elite
Halo 1 Spec Ops Elites (the black ones on the level Keyes)
Halo 1 Spec Ops Grunts (the black ones with Fuel Rod Guns on Keyes)
Halo 2 Green Grunts (they mostly have the turrets, i dont know what the actual rank is, can anyone please tell me if you know?)
Halo 2 Jackals with the orange shield (im not sure if thats a specific rank, probably the Majors tho)
the Halo 2 Navy colored Elites (the darker blue ones)
also the Halo 2 Councilor Elites and Honor Guard Ultras are cool
And like OP said, I thought it was cool how ODST had new and different colored Hunters 
Guta, nuf said.
I’d definitely have to go with either the Elite Councilor or Prophet.
The prophet (San 'Shyuum) are a pivotal role in the game and are, for the most part, very intelligent.
Then the Elite Councilors are just plain awesome.
The classic gold Zealot Elites.
Skirmisher Murmillo’s are definitely my favorite. The combination of speed, agility, good marksmanship, and two arm shields is a really good challenge. However, my eternal love goes to any Sangheili who brandishes their sword. No matter how confident I think I am with whatever empire-destroying weapon I’m carrying, they are the most fearsome foe to counter. These sword-wielding Sangheili are totally ruthless and will stop at absolutely nothing to make their Fruit Ninja skills a reality.
Concerning pure aesthetics, I love the Alpha Chieftain from Halo 3. Their armor represents a wonderful blend of brutal, tribal Jiralhanae culture and the hyper-advanced Covenant. I own few Halo figures, and while I cherish them all, my Legendary Edition from McFarlane is my most prized.
I’ve always been fond of Reach’s Sangheili Spec Ops. I know Reach was far from the greatest Halo, but I loved the Elite’s armors, Spec Ops especially. Something about the helmet is just super cool to me. I do wish their armor stayed black like it had been in the past. On another note, I’d have to agree with several posters above, the Councilors looked epic too.
Halo 3’s Brutes were alright, but their shaved faces could be really off-putting, and power armor looked tacky sometimes. I think I preferred them in 2. But either way, I’ve never really like the Brutes much.
I am very fond of the Alpha Chieftains (Halo 3 Hammer Brutes). Their armor looks so…ornate and tribal…a perfect fit for their Society and Role.
The groomed look of The Brutes in Halo 3, in general, was exceptional. However, the feral look of Halo 2 Brutes was also really cool. Bonus points for Tartarus’ mohawk.
The Skirmisher minors were also pretty neat.
However, there are some designs I hate/think are dumb…
Sangheili Councillors look really dumb. Their helmets are impractical and devoid of function.
Halo Reach Brutes are terrible.
Grunt Designs are flat out terrible in all games.
Unggoy: Ultra and Heavy depending on variant
T’vao Kig-Yar: Commando
Ruuht Kig-Yar: Major
Ibie’sh Kig-Yar: Heavy
Sangheili: Ultra (Halo Reach), Honour Guard and Commander
Mgalekgolo: Hunter
Yanme’e: Ultra (Halo 2A)
San’Shyuum: Hierarch
Jiralhanae: Chieftain
-Faber of Will and Might out
Ungoy: Ultra, cause they have nice helmets and are high ranked.
Kig-Yar: Major
Brute: Brute Cheiftan (Halo 3)
Elite: Elite Ultra (Halo Reach)
Prophet: Regret