The makers of FinalBuilder used to do a survey every year of their (mostly Windows-based) customers. I can't find anything more recent than their 2008 survey, but here's what it looked like then.
As you can see, there isn't just one you have to know. However, you'd be a fool not to familiarize yourself with Subverion and Sourcesafe.
If I were to predict how it has changed since then, I'd guess that Subversion has added users, SourceSafe has lost a bit of ground to Team Foundation, and that Git now shows up, but with only minor numbers.