Not ClearCase, at least for a Unix/Linux system (perhaps with Windows the installer is easier). It was easier for me to learn a new tool, Perforce, rather than upgrading our ClearCase server.

I currently use Perforce at work and I like it but I have no idea if it is the best. Setting up the command line environment and the Perforce Server is a bit awkward, but using the Visual Client is fairly easy. I like to think the users have a fairly easy time with it for everyday tasks; it is just the initial setup takes some work.