Ulfric says Arngeir is the most powerful of the greybeards. But Arngeir says he is the LEAST powerful (hence the only one who can speak to people), so which is it?

Don't know where you heard that, but Arngeir is the most powerful of the 4, except for old Paarthurnax.

Arngeir became strong enough to be able to speak without causing destruction.

And that he is the highest leveled NPC in skyrim at lvl150, haha

And Miraak as well.