Which is the most common malware?

Trojans. According to the recent report by PandaLabs, the percent infections caused by each of the different malwares are:

Trojans (25%)
Adware (23%)
Worms (9%)
Backdoors (4%)
Spyware (3%)
Dialers (3%)
Bots (2%)
Others (31%)

‘Others’ includes Viruses, hacking tools, PUPs, etc.