Although CVS (Concurrent Versions System) is the oldest, it's still the mostly widely used version control system because it's mature, stable, and fielded in a vast corpus of projects. CVS is the standard for comparison among all other VC tools.
Citation:
Panagiotis Louridas, "Version Control," IEEE Software, vol. 23, no. 1, pp. 104-107, Jan./Feb. 2006, doi:10.1109/MS.2006.32