DEET-based insect repellents are the most commonly used anti-mosquito repellent. Brands that use DEET include Off! and Ben’s.
DEET has been around since 1957 and has been studied extensively. It is important to remember that DEET is a repellent, not an insecticide. It does not kill mosquitos; it merely repels them. The chemicals in DEET make it difficult for mosquitos to detect carbon dioxide and other chemicals produced by humans. The main method mosquitos use to find their prey is by sensing chemicals, so DEET essentially makes you invisible to mosquitos.
The best type of DEET product for you depends on how long you are outside and what type of are you are in. If you are just outside for a leisurely walk, you can use one that has a low DEET concentration. However, if you are going camping overnight in the woods, you will need one with a higher DEET concentration. The OFF! website has a list describing their products and their DEET concentration.
Read the instructions on the back of any DEET product before applying. A common misapplication is that people just spray the product in the air around them. This does not work as the product needs to be directly sprayed directly on your clothes and body. After you spray it onto yourself, you should smooth it with your hand to make sure all parts of your skin have some DEET on it. Mosquitos can detect small areas on you that are not protected. Think of spraying mosquito repellent on yourself as similar to applying sun screen in this respect.