He is meek and therefore the strongest. He has great power but holds back in meekness and control. Ergo, he is the strongest.

Strongest. Dragonballz reference: Goku and Gohan were so powerful as ascended super saiyans, that they could be in their transformed state constantly, and not be all glowey glowey. They had better control over their powers, which is why they were stronger and faster than Vegeta and Trunks who had bigger muscles, and a better lightshow display.

What is scarier? The guy who can set off nuke scale explosions, or the guy who can set off nuke scale explosions, and contain them in the palm of his hands?

Arngeir has the power AND the control. He is by far the strongest Greybeard.

Actually, they spoke to you but you didn't die.

That's because Dovahkiin has the guts.
The last dude they spoke to was Tiber Septim, and like the PC, he was named Ysmir that day.

And the guy before Tiber was Wulfhearth, and he was blasted to ashes, like all others who are not deemed the hero.

From UESP:
"Upon ascending the 7,000 steps to High Hrothgar, you will find four Greybeard Elders living in the secluded mountain refuge: http://www.uesp.net/wiki/Skyrim:Arngeir the eldest and most powerful, http://www.uesp.net/wiki/Skyrim:Borri, http://www.uesp.net/wiki/Skyrim:Wulfgar, and http://www.uesp.net/wiki/Skyrim:Einarth. http://www.uesp.net/wiki/Skyrim:Arngeir is the only elder who speaks more than a few words to you, explaining that the others are so powerful that if they were to speak even a whisper, the force of their Thu'um would destroy you. Arngeir may only speak because of his extensive training and age."
There you go.

That's not worth more than our speculations here, until they have another source.