that's up to you. each will have different qualities and sag under its own weight to a point. your span isn't long, it's the weight that matters, and how you're supporting it.

in most choices I'll choose whatever plywood looks decent/ decent priced I would stay away from 1/4", but if you're just stacking some paper towels it'll work. I use 3/4" minimum but 1/2" will work great for most people's needs. if you face it with 1x2" material and support on 3 sides you add significant resistance to sag.