Best weed barrier (non spray)?

Cardboard truly works and it will last all year! The trick is to take all the mulch off the bed/area where you want to block the weeds, lay out a layer of cardboard and wet it, then top it with mulch. Of note: if you don't soak it, the cardboard will just dry and curl up, moving the mulch on top along with it and it will look messy.

The cheaper, most basic brown cardboard works the best! When making a new bed I like to use cardboard broken into rough 8 X 11 pieces much like weed barrier fabric, plant my annuals/perennials in between then mulch on top. You don't want gaps in between the cardboard because some weeds like bindweed will seek out that tiny gap and grow... But it really really works!