It depends upon the kind of projects you are working on. PVA glue is one of the popular adhesives when it comes to joining paper or cardboard. It is a strong and inexpensive solution. However, if you are making something structural with the cardboard, you can go for E-6000 adhesive, an industrial-strength adhesive. You can use it for cardboard and also other materials like metal, wood etc. Spray adhesive is another option if you want to cover large areas quickly.