Choosing an OS for anyone isn't a very easy task. There're pros and cons to everything in life, and OSes is no different. I'm one of those few people that use many OSes on a daily basis, and as part of the open-source community I do look at, write, and review code from time to time. That's not to say that I look at the source of every single package I install; rather, I take a look at the code of that which I believe is necessary. Major open-source packages get reviewed, and when someone violates the trust of others their software is quickly flagged and removed. If the perpetrator isn't the maintainer, then, they're quickly blacklisted from other projects and their reputation goes to the crap bucket. One of the reasons why people don't add malicious code is because "we" (i.e., members of the open-source community) will hunt you down. We won't hack you because we operate within the law, but I guarantee that no matter how good you are at tunneling and hiding yourself, we'll find you and you will be brought to justice. This results in programmers losing their jobs and completely losing any reputation. So, for this reason, people don't intentionally try to do bad things when they know of the consequences (i.e., never getting hired as a software engineer again).
Now, Windows is as good as OS X, Linux, Unix, and other Unix-variants. Each OS is good for something, and each OS is better suited at certain tasks than another. If you want to know the most secure OS out-of-box, the answer is probably OpenBSD. If you want to know the best OS for gaming, it's definitely Windows. Or, if you want to know which OS has the most beautiful GUI, the answer is likely OS X. If you ask which OS is the best for embedded systems and frugal people, my answer is Linux.