First splatbook you begged your DM to approve?
I don’t think we ever had to ask the DM for permission. If it was published by TSR it was gospel and if someone brought it in, we used it. The only real splats I remember using where the racial and class books. Anything with new spells were really popular.
The one I wanted most, but never got was Vikings.The only one I ever owned myself, (and still have is) Villains, though I can't remember why.