There is no conclusive answer to this question as the most common type of malware varies depending upon which region of the world you are situated in, and also changes over time. https://cleardex.io/?p=872 Some of the most common types of malware consist of viruses, worms, Trojan horses, and adware.
Infections are among the earliest and most common types of malware. They are programs that are designed to reproduce themselves and infected other computers. Worms resemble infections, but they are able to spread out without needing to connect themselves to another program. Trojan horses are programs that masquerade as something else, such as a video game or a genuine program, but are in fact destructive. Adware is a kind of malware that displays advertising on a computer system, often in a pop-up window.
Malware is constantly progressing, and new types are constantly being produced. However, the types talked about above are some of the most common that you might encounter.