<p>There are many SCM systems out there. Some open, some closed, some free, some quite expensive. Which one <em>(please choose only one)</em> would you use for a 3000+ developer organization with several sites (some behind a very slow link)? Explain why you chose the one you chose. (Give some reasons, not just "because".)</p>

Any DVCS (BitKeeper, git, Bazaar, Mercurial, etc) because being distributed will cut down the load on the central 'canonical' SCM server.  The caveat is that they're fairly new technology and not many people will be familiar with their use.
If you want to stick to the older, centralized model, I'd recommend Perforce if you can afford it, or Subversion if you don't want to pay for Perforce.  I'd recommend subversion over CVS because it's got enough features to make it worthwhile but is similar enough that devs who already know CVS will still be comfortable.