First of all, there are the four D's: the Defiler, the Deceiver, the Destructor, and the Darkener. In the Greater Legion, it will actually be the five D's, for the Daughter of the Moon shall rank below Archimonde and Kil'jaeden but above Mannoroth and hence also above Tichondrius (Tichondrius and Mannoroth treat each other more or less as equals).
Anetheron and Mephistroth speak with Tichondrius on equal terms (though we know that Tichondrius is technically the cheif among the Dreadlords), so they should also count as major.
After that, it gets iffy. It's hard to convey what exactly I mean by 'major,' but it should be someone important enough that there shouldn't be easy replacements. Example of those who are not major: Mal'ganis, Varimathras, Detheroc, and Balnazzar. These guys are lackeys, and I wonder if they are "Lords" at all (though of course they belong to the Dreadlord species).
Since we have at least three major Legion Lords of the Nathrezim, who tend to be weaker on average than the Eredar or the Annihilan (the Eredar and the Annihilan mobs are, as far as I'm aware of, always elite mobs in TBC, while there are many non-elite Nathrezim; and also some of the Nathrezim serve Illidan, suggesting that they are not as stalwart as a species), there should probably be as many or more major Eredar Lords and Pit Lords. Of course, not all of them need to be among the demons already known to us.
Among the Pit Lords, Azgalor is a clear candidate. He appears alongside Anetheron at Mount Hyjal. Antheron is replacing Tichondrius, while Azgalor is replacing Mannoroth, and since we have already established Anetheron as a major Lord, it seems reasonable to suppose that Azgalor was also a major Lord. On the other hand, it is hard to imagine Rage Winterchill, who also appears at Hyjal, being considered a major Legion Lord (and I would be wary of granting Kaz'rogal a major Lordship status either); and given that the WCIII manual states that the Pit Lords were fanatically loyal to Mannoroth, it seems like the difference between Mannoroth and the second most powerful Pit Lord would have been greater than that between Tichondrius and Anetheron (the second most powerful Dreadlord), who have a more brotherly relationship.
There is also Magtheridon, who is called Lord of Outland. He is deposed of his position by Illidan Stormrage, but that is okay since Illidan, after having consumed the Skull of Gul'dan, deemed himself "more powerful than any of Archimonde's lieutenants," and indeed proceeded to slay Tichondrius, who was doubtless major. But later he is even enslaved; it's hard to see a truly major Legion Lord allowing this to happen. Moreover, the basic issue with him is that he seems to have no long-term vision, content with ruling the Black Citadel (Black Temple); Illidan even calls him "complacent." I would be uneasy if the Legion had to rely on his leadership, regardless of his destructive powers.
Then we have Brutallus. He rather easily slays Madrigosa, but it is otherwise hard to judge his power. He is presumably a trusted lieutenant of Kil'jaeden's, for he appears at Sunwell Plateau to herald his coming.
Among the Eredar, we know that Talgath was a trusted servant of Kil'jaeden's, ever since they first joined the Legion on Argus; he is also used to personally spy on the Orcs on Draenor. It seems hard to ascertain his rank with the information available.
Jaraxxus calls himself "Eredar Lord of the Burning Legion," implying some sort of lordly position within the Legion. There can hardly be any doubt that he was rather powerful Warlock; however, it seems rather unlikely that he was in any major leadership position, or a being of tremendous magnitude. Unlike Archimonde or Kil'jaeden, he requires no special artifacts to summon; indeed he is summoned by a "trifling Gnome." And he walks in alone without any guards or army at his side, which is not the way of the Lords.
We have too little information about Lady Sacrolash and Grand Warlock Alythess to conclude one way or another. But we could probably say that they were trusted lieutenants of Kil'jaeden, and that they were probably as powerful or more powerful than Sathrovarr or Brutallus.
Prince Malchezzar sits atop Karazhan, the tower in which Sargeras himself had abode, and makes quite grandiose claims about himself, including the ability to manipulate time. He does not seem involved in any large-scale operations however and is unlikely to be particularly high in the hierarchy.
Going back to the Nathrezim, Sathrovarr surely deserves to be taken seriously. Balnazzar, Mal'ganis, and Detheroc possess mere humans; Sathrovarr possess a Blue Dragon, and a particularly potent one at that: for Kalecgos is the one who will later replace Malygos as the Blue Aspect.
A special case is Hakkar the Houndmaster, whose race is uncertain. He is clearly below Mannoroth, but probably important enough; he is one of the only three named lieutenants of Sargeras during the War of the Ancients.
As for the other, lesser races, only the Ered'ruin have given us anyone worth considering: Kaz'rogal and Kazzak. Kaz'rogal is present at Mount Hyjal along with Azgalor and Anetheron, but I consider it unlikely that he was as highly ranked, for we know that the Doomguards were mainly Archimonde's personal bodyguards, rather than commanders in their own right. Kaz'rogal may be the chief of the bodyguards, but not someone capable of leading a war by himself.
Kazzak is styled 'the Supreme,' and shows more independence, staying behind in the Eastern Kingdoms, and re-opening the Dark Portal to get to Outland. However, what he actually accomplishes in Outland is rather uncertain. Kael'thas seems to have done much more actual work.
So, my tentative guess for the Legion hierarchy (among the known ones) would be:
Sargeras
Archimonde & Kil'jaeden
Azshara
Mannoroth, Tichondrius
Anetheron, Mephistroth, Azgalor, Sacrolash, Alythess, Talgath
Hakkar, Sathrovarr, Brutallus, Kazzak
Jaraxxus, Malchezzar, Magtheridon, Kaz'rogal
