[A_Brandybuck]

There are only few realms left in Wilderland. We have first the Dwarves of the Iron Hills under Daín, as you mentioned. Second, we have the realm of Thranduil in Mirkwood. Furthermore, there are the Beornings, but they could hardly considered a realm at this time, right?

Southern Wilderland is deserted. All people moved northwards after the wars with the wainriders etc. The Éothéod moved then southward to settle in Calenardhon. As realm of Men could only be considered vaguely the people of Laketown and nearby

So, I would say, that only the Elves of Mirkwood and the Dwarves of the Iron Hills could be considered as realm. When it is told, that only the Dwarves of the Iron Hills could stop Sauron from taking back the old realm of Angmar, then I would say, that it is meant, that they are strategically able to stop Sauron, because of the location of the realm. I am sure, that the Wood-Elves could stop his army, too, if they have come through Mirkwood.

But sending his army through Mirkwood or along its borders would mean to face the Elves and probably the Dwarves together. Destroying only the Dwarves of the Iron Hills would mean, that they could reoccupy Angmar going through the northern waste lands and avoiding Mirkwood and therefore the Elves.

For that reason, I would not say, that the Dwarves are the most powerful realm in Wilderland. The Elves could (only could) be more powerful or equal.