Well... not to rain on the redguard love parade since I actually like them a lot as well, but...
They're a bit overhyped. They didn't beat the Dominion, they simply held them off long enough that the Dominion gave up. Impressive, but largely thanks to the legion that stayed behind against orders to help fight the dominion back when they overran Hammerfell.
The other comment about holding off against Imperials Nords and Bretons is a bit off, since back then, there was just cyrodiilic Nords and Bretons. But, they indeed did hold off for a good while, only losing fully because half of them sided with the empire.
But their most impressive feat people mention is when they took Hammerfell in the beginning from the other elves and humans fighting over it, except that the forces were small, because Hammerfell was barely populated since they couldn't handle the harsh environment, and they did this with a surprise attack from their entire warrior class...
But Redguards despite that are impressive. They sunk their own continent after all, and even fought amongst themselves just to decide who would have the honor of leading the slaughter of the left handed elves... bad ass.
My favorite is Nords though. They have some of the most powerful magic in the lore with the most varied and interesting effects. That being the thu'um. (Not counting Dwemer magicks, which are unknown to us unfortunately.) They defeated an enemy supposedly more intelligent and powerful than they were and took their land back after just the destruction of their one city, and their lore is so out of there and strange, its hard not to love. Like 911th cow, and all those weird ass names the companions had.
I know you weren't talking about why we like them, but I felt I should mention it. They did manage to defeat a combined Redguard and Breton force over Dragonstar, and they defeated the dragons. They had the first human Empire over Bretons and Dunmer, and they've had the first dragonborn and shezzarine.
Burd, Nordic Captain of the guard in Nordic Bruma, was so badass, that we had to ask him to stop closing gates so much so that the enemy could summon their great gate. So much for the mighty Daedra.
But if we're going to argue over credentials, the Akaviri are likely the most powerful.