There is a wide range of materials that can be used for undershirts, but the most popular and widely-available options are cotton, polyester, and rayon. Each of these materials has its own advantages and disadvantages that should be considered before making a purchase.
Cotton is the most breathable option and will keep you cool and comfortable even in warm weather. However, it is also the most absorbent material, so it will retain sweat and body odor more than other fabrics. Cotton undershirts are also more likely to shrink or stretch out of shape over time.
Polyester is a synthetic fabric that dries quickly and doesn't absorb sweat like cotton does. This makes it a good choice for people who tend to perspire heavily or who live in hot climates. However, polyester can be less comfortable than cotton next to the skin and is not as breathable, so you may feel hotter in summer weather if you choose this option.
Rayon is a natural fiber made from wood pulp that has many of the same properties as polyester. It dries quickly, doesn't absorb sweat, and is light and airy enough to keep you cool in warm weather. However, like polyester, it may not be as comfortable as cotton next to your skin.