I would say SourceGear Vault. We've just switched to it after using VSS for many years. I haven't looked back once. Vault integrates with VS if you want it do (I don't - I now prefer the CVS edit-merge-commit method), is SQL-server based, has fully atomic transactions, excellent branching and merging support, proper ACLs for projects, and more. And no, I don't work for SourceGear! :)