This one is easy. The one type of die I already own that I dearly love is the humble d12. The one type of die I do not own, but wish I did is the d24.
From an RPG math standpoint, 2d12 rolling against a difficulty score hits that sweet spot for me in having easy to understand difficulty numbers and a bell curve. (Yes, it is a triangle, not a curve.) Something for me is easier to imagine that difficulty moves in 4% increments and that a DC of 25 is technically impossible without some help from a bonus.
From an aesthetic standpoint, I like pentagons. I also like the way it rolls due to its round shape. Holding a handful is almost like holding marbles.
In the same vein, a d24 is like hold a big Cat's Eye marble.
Oh yes, I would buy a pound of d12s if I could afford it. One day, I'll purchase some d24s just because.
Tomorrow, maybe another petty little post. 🙂