I'm using UCCNC. A lot of former Mach3 users have moved to it, and find it a much better product. New features are constantly being added.
Many like the Centroid Acorn, claiming it's a more "industrial" solution.
LinuxCNC is good, if you don't like Windows.
There are other options, some more expensive, some cheaper.
At the end of the day, it's a matter of personal preference. Does it do what you want and need it to do. Some may meet your needs, others may not.