Eh, I honestly don't see either of them doing a good job. Paarthrunax would almost certainly fall prey, at least a little, to his Dragon wiring and become corrupt.
As for Gelebor, I don't know much about his character seeing as I still haven't gotten that far into Dawnguard (I know I need to start playing more often ), but the fact he's an elf, and a snow elf at that, makes me skeptical that he'd be able to rule over the descendants of the people that were his mortal enemies at one time without being bias and abusing his power for even a small measure of "revenge".
Honestly the Nords, IMO, deserve a Nord leader. However, they deserve a Nord leader who has a level head and can make decisions that would benefit Skyrim and his people in the long run.
At this time I don't see anyone that really fits the bill perfectly, but Balgruuf is probably the closest they've got.
In a perfect world though ol' Uriel Septim VII would come back from the dead and in Patrick Stewart's amazing voice tell everyone to unrustle their jimmies and go to their rooms (or provinces of origin... whatever).
You will be shocked when you meet Gelebor, i won't say more as to not to spoil it for you. As for dragon-yoda, the fact he knows he's not perfect and can again fall to his old habits, and tries, very hard, each day to overcome the negative side of his nature is why i choose him. It really makes me laugh, but aren't humans the same? do they not get corrupted, become inmoral bastards only seeking to control, dominate and profit? Paarthrunax at least tries and was one of the dragons who turned against Alduin and aided the ancient nords. Some nords sided with Alduin. It isn't the race or species that determines what you are or what you believe in, it's the decisions you make and Paarthrunax has stayed true to his ethics through a very long time, something many people can't boast of.
It's true that race doesn't always determine someone's fate or beliefs, but it's far more common that it does. While there are many people that overcome their upbringing and become something more many others don't. Paarthrunax tries to overcome it, but as a dragon he's still prone to fall into the same habits and even he, as you mentioned, knows this which is why I don't think he'd be able to be a good leader (he is an amazing example of willpower, but even he knows he's flawed which is why he hides away).
As for humans, yes they are the same. Profit, power, etc. are motivators for every being that has ever existed. I'm not saying the Nords are perfect, far from it in-fact. I'm simply saying that they should lead themselves as all the other races do.
I understand your point, it can only be a nord leader because the nords would only follow a nord or human.
Still i think Paarthrunax is being too hard on himself, after Alduins death he wanted to spread the way of the voice to the other dragons, you have to admit that it would've been the perfect time to take Alduins place. But that intent was never his intention, he has proven to be wiser and exercise more restraint then any other being in the game. By the way the nords aren't the only inhabitants of Skyrim...
True, Paarthrunax is an all around cool dude. And honestly he may not (and there's a good chance he wouldn't) become corrupt, but I don't think I'd be willing to take the chance. Dragons are just too powerful IMO.
As for Nords not being the only race in Skyrim, no they're not, but they are the majority by a HUGE margin. There were other races in Morrowind too, but Dunmer were the majority and it was their land so they ruled. Cyrodiil is full of members of every race, but the Emperor is an Imperial (I know the Empire has had leaders that weren't Imperial in the past, but lets be honest it's rare and hasn't happened in -forever-) because Cyrodiil is the seat of -their- Empire.
It's never actually stated, but it is implied. All artwork and advertising by the creators of the series indicates that the Dragonborn is indeed a Nord in their version of things which is as close to canon as we can get.
The Nerevarine is also never actually stated to be Dunmer, but given the hatred toward non-Dunmer in Morrowind, and the fact that the Nerevarine was the reincarnation of Nerevar I'd say it's safe to assume he was a Dunmer. At least in Bethesda's eyes.
As for the Hero of Cyrodiil, it's actually mentioned in-game that the current Sheogorath is probably the Hero of Cyrodiil from Oblivion, and from his appearance I'd be very comfortable assuming he's Imperial. Now it's possible that he changed his appearance to that for some reason, but honestly that's just nitpicking.
All in all I guess the heroes could be any race you want them to be (both in-game and in lore) which is probably Bethesda's intention, but there's no denying that those races just make far more sense and probably were the races Bethesda was thinking of while creating the games.
I would never put Dovahkiin on Skyrim's command. Never.
I think that the best candidate is Jarl Balgruuf. He seems prudent, is a "True Nord", and he has two good advisors, Irileth (Dunmer) and Proventus Avenicci (Nibenese, I think), in which he trusts.
So the fact that his two main advisors are from two different races, only supports that Balgruuf would be a good choice, because at the same time that it would be a government with diversity, it would be a traditional one. (At least superficially, but that's enough to keep those true sons and daughters of Skyrim happy and quiet).
[EDIT] Also, Balgruuf has at least one heir that seems as good as the father, Frothar.
But the advertisements -are- as much a part of the lore as the actual games are. You may not like it or agree, but you can't deny that Bethesda does in-fact see a Nord as their Dovahkiin.
As far as I'm aware the only difference between Chimer and Dunmer is the red eyes and grey skin which was a curse or something. You say that the Nerevarine isn't a clone or child of Nerevar, and you're right, but he is the reincarnated Nerevar and almost every fantasy thing I've seen that involves a reincarnated character has that character come back looking something like the original.
Also I'd argue that the current Sheogorath -doesn't- look like the old one. He's got a very short beard, much longer hair and pure white eyes. He also seems much more gaunt and old/sickly. The old Sheogorath had cat-like eyes, very styled and slicked hair and a much fuller beard. Again, yes he's capable of changing form as a daedric prince, but look at the "default" character in Oblivion. Long haired Imperial. The current Sheogorath very much looks like an older version of him.
Aside from that I'd also point out that when you actually start the games you are given those races as the default. Morrowind=Dunmer, Oblivion=Imperial and Skyrim=Nord. It's pretty easy to see that those are the races that Bethesda used as the template for their respective games' heroes.
It's also noticeable in quests and interactions in-game. In Morrowind you could join any great house you pleased despite your race. This, however, doesn't seem likely as all but house Redoran seemed to have a pure hatred for outsiders, non-Dunmer in particular, and even Redoran didn't seem to like them. In Skyrim you're able to join the Stormcloaks and enter Stormcloak cities despite being any race you choose. Now while there are members of most races in the cities of Skyrim it doesn't explain your Khajiit dovahkiin being able to enter Windhelm or your Altmer being allowed to join the Stormcloaks.
All this can be explained away with "Oh it's game mechanics", but that's just nonsense IMO. Yes it'd be easier to program without those things, but you have to think that Bethesda foresaw this and designed the game to be seen from that races perspective. They could have easily not allowed you to join the Stormcloaks and stuck to lore, or denied you the ability to join the Great Houses. Hell that'd have been easier than creating the factions' quests in the first place.
I'd also like to point out that all the preview material of Skyrim showed a Nord as the character being played. Should show that the developers too favored the idea of a Nordic Dovahkiin.