Most vulnerable operating systems and applications (including browsers):
7,038 new security vulnerabilities were added to the National Vulnerability Database (NVD) database in 2014, an average of 19 vulnerabilities per day. The number is significantly higher than in 2013 and continues the ascending trend over the past few years. Over 80% of the reported vulnerabilities are in third-party applications. Operating systems are only responsible for 13% of vulnerabilities and hardware devices for 4%.
Top operating systems by vulnerabilities reported in 2014
Yep, Windows is not the most vulnerable. Apple with OS X and iOS is at the top, followed by Linux kernel.
Top applications by vulnerabilities reported in 2014
Web browsers are obviously the most security vulnerabilities because they are a popular gateway to access a server and to spread malware on the computers. Adobe Flash Player and Java still vulnerable. It's better if you can avoid them, or keep them up to date.