Noobie here. What is the Best Diy material for the best possible picture?
Dependent upon the size of the screen desired,
SINTRA could be considered the topmost choice for any screen's dimension that does not exceed 120" wide for 2.39:1 and 107" wide for 16:9 Smooth, because it is ready "as is" to accept any good DIY oriented coating
Beyond that,
Flexi-White Vinyl Fabric for Frame-built screens is a very plausible 2nd choice.
If a material is desired that can be construed as being "Ready to use as is", it being a ultra smooth, 1.2 gain white surface will allow it to suffice if a white surface can be used., It is also easily painted upon after it is stretched onto it's frame.
And good 'ol
Drywall finished to a smooth surface is a easy 3rd choice. The latter coming with almost no size limitations save the size of the wall itself....and the condition of the surface one has to deal with at the start. It's surface can be as good and as accommodating as you can make it....painted with almost any imaginable DIY Screen coating....after Priming of course.
There are lots of other choices....some very suitable and inexpensive when screen size drops below that which can be accommodated by any material 4' x 8'
Some might dispute the choices made above, based on preference, personal experience, or simply to be cantankerous.
But the above have all been vetted via a considerable amount of experience by many DIY'ers, and the first 2 are staples for consideration by those aspiring to create advanced screens that have the best possible performance.