Obviously, "best" is meaningless without a specific set of criteria by which to judge it.
But it would be interesting to know what Revision Control systems people use, why they chose them, and what they think of them.
I've used SVN for years, with Tortoise SVN as the Windows client.
It is free and seems to do all I need without much fuss.
I use an off-site repository hosting service (not free - but not expensive)
It is widespread among clients, but I note that Git is gaining popularity - so I'd be interested in any experiences of the relative merits & demerits of the two.