Elites - Elites are badass. I love Commando and Flight armour.
Hunters - I used to be obsessed with Hunters. My old gamertag (my old account, I had to make a new acc, child account issues) was "lekgolo197" named after the Lekgolo worm (Hunter = MgaLekgolo)
Grunts - Fun to headshot (Grunt birthday party FTW!)
Jackals - They've got good aim, fun to fight, slightly evasive.
Skirmishers - "DAMNIT! COME BACK HERE YOU EVASIVE BASTARD! LEMME MELEE YOU! ARGHHH..."
Drones - It's fun to mow down buggers on Halo 3 with an AR or an SMG
Covenant Cruisers - Friggin' epic. Huge, aesthetically beautiful, great functionality - I'd love to live in one.
Phantoms - Pretty epic. Hard to decide whether or not the green Heretical phantom is better than the purple Covenant phantom, they're both epic.
Dropship - Fun to wait by one on rocketfight, then shoot a bunch of Covie's upon their drop.
High Prophets, Lesser Prophets, etc. - Great system in place, some of the prophets piss me off though. Heh, "I am truth! The speaker of the Covenant!" "And so, you must be silenced..."