Depends how much detail you are going for, how long you want the stencil to last, what you are spraying onto, what you are spraying with. You mention street art, so I’ll assume you are using spray paint in a rattlecan, on hard-ish surfaces like concrete, bricks, or walls.
For one-time use stencils, adhesive stencil material is amazing… but it ain’t cheap.
I did this the other day by accident with some regular paper masking material.
Cardstock, 80-110lb conform a bit to surfaces, can handle some detail, and can last for a couple dozen sprays, or more if you are light on the trigger. It’s hard to clean without destroying, and so as the paint builds up, you lose details and the page curls. It will last longer if sprayed alternately from either side, if your design works backwards as well. It’s cheap, so it doesn’t hurt as much to throw them and run, lose them, tear them, or hand them out to a street team.
Manilla folders are good too, if you are raiding a supply cabinet, but more expensive to purchase in bulk. The finish on them makes it a little easier to wipe clean for a little longer.
Thin cardboard/pressboard/cereal boxes hold up a while, but they don’t take detail too well. Great for big blocky shapes, big stencil-fonts, and basic curve/corner templates.
Mylar is nice, doesn’t tear, and when you have built up a thick enough layer of paint, you can just bend and twist it to crack the paint away. More expensive than paper-based stencils, and will last much longer, but not indefinitely.
The Lucent by Rowmark product looks like it will be a step up from mylar, but I have not yet tried it out.
Thin Plywood and thicker cardboard work ok. If you are doing road-work sized stencil-font letters, they will work out fine. Neither will conform to surfaces, though, so you have lots of opportunity for underspray and fuzzy edges. Again, good for blocking out larger sections, and plywood will last ages.