What is best for you depends on a few factors, but most medium to large projects CVS.
Bitkeeper is also one to consider for some projects.
It's used for the Linux kernel even though it has a restrictive license.
Clearly Linus would have had to see some significant
advantages in Bitkeeper over open source tools like
CVS for him to force the kernel developers to use it.
This page gives and overview of many options:
http://www.linuxmafia.com/faq/Apps/scm.html
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