My two packages that get used pretty much all the time are biblatex(-chicago) and memoir. Memoir is my friend because it includes so much of the functionality that is otherwise spread across a bazillion packages, and makes sure it all works nicely together.
I'm using KOMA-Script (scrbook, etc.), although I looked at Memoir at some point. Both have very detailed documentation, and I don't have strong reasons for choosing one over the other.
I have a sense that the coin toss between KOMA and memoir often comes down to which direction, east or west, provides the shortest route to the Atlantic Ocean.
