The best diff tool for OS X is IntelliJ? (jetbrains.com)

I like KDiff3 for osx.

http://kdiff3.sourceforge.net/

Ugly, but just works :)

I've always been pretty happy with Diffmerge

https://sourcegear.com/diffmerge/webhelp/sec__inst__mac.html

Instructions for commanline use are pretty easy to find as well as setting it up as the default diff toll for git and svn