The best holes I've ever seen are the single hole electron microscope grids I used in a variety of transmission electron microscopes. These apertures are made by etching and are close to perfect at a molecular level.
The sharpest looking pinhole pictures tend to exploit high contrast high frequency detail that is just coarser than the limiting resolution of the pinhole itself.
Sharpness in pinhole images is limited by diffraction at the edges of the pinhole. The triangular pinhole has greater edge length in relation to the opening area than does a circular pinhole. However, there is one advantage to a triangular pinhole; by moving only one blade the aperture can be varied. With more care, a four-sided variable pinhole can also be constructed.
Originally Posted by Mike Walker
I'm no expert, but I was talking to a man recently who made large format pinhole chromes, and he used acupuncture needles of hair thickness. IIRC he used taught metal foil.