That's funny. Of all the Planescape books, I think I referred to "On Hallowed Ground" the most of any of them during the Planescape games I ran. I think it's probably because I ran a lot of adventures that involved the various powers and their realms and found "On Hallowed Ground" as a good reference and inspiration for that.Hmm, I have On Hallowed Ground, and it's probably my least favorite PS product. I think it's because I was expecting it to be a PS-themed update of Legends and Lore and I was very dissappointed with it, because it's almost 100% fluff. Some of it's ok cause it gives a lot of background on planar cosmology and who the gods interact, but I don't think I've ever made much real use out of it.
After "On Hallowed Ground", the PS reference I drew on the most for inspiration was probably "The Factol's Manifesto".
If I had to pick just one of those two books, I'd probably go with the Factol's Manifesto because it captures much of what I think is the core spirit of Planescape.