Devil (Duke), Gorson
Also known a the Blood Duke, Gorson is a fiendish wemic-like duke (for those of you that aren't familiar with the wemics, they are a Forgotten Realms creature that resembles a centaur with lion, rather than horse, features) that commands the ranks of the barbed devils. The CR 22 Gorson is heavily melee combat-oriented, with pounce, rake, improved grab, and a +3 wounding battleaxe as his weapon of choice. Of course, being a duke, he does have SLAs as well, but he's definitely more physical than most of his kin.
Devil (Duke), Hutijin
Hutijin, a CR 22 Duke of Hell (what a shock there), is the commander of the pit fiends; indeed, he himself resembles a slightly oversized pit fiend, and it's not unreasonable to imagine that he is merely a pit fiend that got lucky or worked really hard. In addition the standard dukeish traits, Hutijin has the ability to induce fear by speaking and wields not one but two unique tool: a +4 unholy flaming burst trident and a net of snaring.
The big man himself, Lucifer was the original creator and ruler of the Nine Hells. One of the three main supreme evils of the planes, Lucifer was eventually deposed by one of his 9 lieutenants: an upstart archdevil by the name of Asmodeus. Asmodeus and seven other archdevils lead the Great Uprising against Lucifer and his lone loyal lieutenant, Belial. While the fighting was great, numbers and Belial's eventual betrayal cost Lucifer the war, and Asmodeus ousted him from Hell. Not one to be spurned so lightly, Lucifer sacrified some of his own essence to build a plane of pure hellfire, malice, and epic level evil known as Infernus. There, he still waits, gathering up power to attempt to take on Asmodeus and his fellow conspirators for his rightful throne once more.
One hell of a character, Lucifer is a whopping CR 39, making him the most powerful being in the entire ToH. Lucifer is not one to taken lightly, as he has a veritable boatload of abilities to back up his tremendous Challenge Rating. He has acidic and venomous secretions, two gaze attacks (one that acts as an alignment shifter, one that is a double-strength bestow curse), an absurd amount of powerful SLAs such as meteor swarm (with all but one, wish [which is 1/day], being at-will), casting as a Sorcerer 20/Cleric 19, the ability to summon archdevils - albeit a suspended power which he can't utilize unless he's back in Hell - as well as standard devil summoning, regeneration and planar omniscience like most major devils, and his key weapon, the Rod of Infernus. This mighty rod works as a +6 greatclub which has the abilities of a rod of rulership, plus a blast of utter annihilation that only deities can reverse, 3/day use of trap the soul, and extreme electricity damage and level drain to any non-devil that so much as touches it. This is definitely the end-boss of any epic level planar campaign, unless you're shooting for uber-deities as well.