The age old question "Which OS?" or "What OS are you running?"
Ok. It's real simple there is no better OS over all. It varys from person to person. Most people like Windows because they just want internet, gaming and that sort of crap. Networking most people go with Linux or Unix. Really if you want to have any grasp of knowledge about compiling or networking. You need to learn Linux. It's very simple but most people don't realize. All you really have to learn is Red Hat or Fedora. Because most of all Linux distos are based on it. Once you learn one of those. It's not gonna be really hard to figure out the others. But still you're gonna want to try all of the Linux distos to see which one fits you bested. Some like Slackware, Some like Knoppix.
Personally I run Windows mostly, but Linux is always somewhere on an old computer I have.