Since glue or some type of adhesive is required to attach materials to a base, watercolor paper is perhaps one of the best choices. This is mainly because watercolor paper won't become overly wrinkled after an adhesive has been applied. As a result, the collage should have an even texture free from bumps or other inconsistencies.
Another option for a collage base is traditional poster board. Part of the appeal of this material is that it's inexpensive and readily available at many retail and grocery stores. It also comes in a variety of colors, which can be beneficial when trying to achieve a certain theme for a collage. The main drawback is that it can sometimes create wrinkles on a collage's surface once an adhesive has been applied.
While canvas is a base that's commonly used for paintings, it can also make for an effective collage base as well. Perhaps the biggest advantage of using canvas is that it's solid and durable and is easy to hang on a wall once a collage has been completed. In addition, using canvas often provides a more professional look than other bases.
Cardboard is an additional option that some collage artists prefer. Some advantages of cardboard are its inexpensiveness and stability. In many cases, artists can get cardboard for free by cutting up a cardboard box. Another benefit is that it can be cut into different shapes that can give a collage its own distinct look. The main disadvantage is that wrinkles or bubbles sometimes show up after applying glue.