I have cut a LOT of stencils over the years (from 6mm lettering to 1.5m lettering), both by hand and on machine. Without knowing the application (flat surface, curved surface), what thay will be used with (solvent paint/ink, water based paint/ink, catylized paint/ink), and how the will be used (hand painting, spray, tracing, etc), it is tough to answer, but, honestly, pretty much any fairly stable material you can cut is fine.
@JaredHooper suggested HIP, and I concur for some applications, but it isn’t suitable with many solvents.
HDPE sheet (milk bottle material) is pretty god with most any solvent and cleans easily, but can be tough to hold to cut in thin layers (and this is based on cutting with a knife, not a rotating cutter). PVC sheep can be ok as well, and is easier to hold, as long as it won’t see solvents it is sensitive to.
For minimal curvature applications, stencil board is good (it is a thin, stabilized brownboard product). For larger work, 1/8" (3mm) to 1/4"(6mm) plywood is good. We use 1/4" luan underlayment for 450mm to 1.2m stencils when laying out commercial ships.
I have seen acrylic (perspex/plexiglas) used, as well as lexan/polycarbonate sheet, though it is expensive and can snap. Both are also subject to edge issues (melting and microcracking) from heating by the tool.
For most applications, I would avoid metal. It will last, but if a metal stencil is bent or kinked, it is pretty much not repairable. HDPE can be flattened out, stencil board can be flattened and taped, etc.
I have stencils that have been around for years that were cut from manila folders, and they have been used for hundreds (thousands?) of applications (we do repeat work for several shipowners, so I made up decent ones for common tasks like owner marking). Size the material (I spray lacquer followed by shellac if needed to protect from solvents) and they last pretty much forever.