I had used Microsoft VSS for years and now use TFS, simply because those are the source control systems supplied by the company with which I work. Both of them integrate directly with TestComplete.
However, I have done some good work with Tortoise HG (Mercurial for Windows) and REALLY enjoyed it. The distributed system of it really helped a lot with being able to work collaboratively on the test project. It does not integrate directly with TestComplete so there are processes and SOP's that you, as an organization, would need to work out to make sure that code integrity is preserved and that everybody stays up to date with the latest source.