That's very subjective. Ask a hundred people and you'll get a hundred answers. It depends alot on what you are wanting it to do. I used a number of CAM systems over the years and there were good and bad to all of them. But once you get familiar with one, you can make it do almost anything you need it to do. I am using MasterCam now. It is not the best CAM program I have used but it's pretty darn good. The more I use it and the more features I learn to use, the more I like it. I've not used OneCNC but from some of the posts here it seems to have a loyal following.
If you haven't invested in anything yet, maybe you should look at some of the other packages as well. One of them may be more suited to your needs, again depending on what you want it to do. A friend of mine has been using EdgeCam for a year or two and he swears by it. From what I've seen of it ( and my limited hands-on time with it), I think I could get to like it alot.
Modern Machine Shop magazine has an online CAD/CAM selector that helps you narrow down your choices based on your criteria.
http://www.mmsonline.com/cadcam/cad_selector.html
There is also a good article that explains the differences between a Geometry-Based and a Process-Based CAM system. Understanding these differences may help influence your decision as well. Read it here:
http://www.mmsonline.com/articles/040302.html