I'd say the Bjerrum length. It's a nice comparison of thermal and electrostatic energy and gives you the minimum length a force must act at.
I think that many of the formulae (is this the plural of formula?) that were covered in the book were fundamental in describing the phenomenon that they described, but were not fundamental in other situations. So I think that the van't Hoff relation was the most important, as it appeared a couple of times in the chapter, whereas the others only appeared in their on section.
i think that with the focus having been on osmosis, the van't Hoff equation would make itself a prominent equation of this chapter.likewise though, the Bjerrum equation and Poisson-Boltzmann distribution are also two important equations