“Best” is subjective and you’ll get several different answers
Versions and SmartSVN are used a bunch
Assembla (folks who will give you a free svn repo) have a list
http://svn-ref.assembla.com/mac-svn-client-reviews.html
I use one that is the exact same on OS X, Windows & Linux as it’s java based
UI is ugly as sin but it IS portable which means what I’ve learned about it on OS X works on Windows & Linux too