I've created this poll so you can vote on your favorite word processor for Mac OS. I've only included software which allows advanced formatting of documents (simple writing software, script writing and outliners were not included).
The question is not practical unless you state what you plan to use it for. "Best" word processor for one user is not the best for another.
MS Word I think is the best for OSX, I've used Pages but I find Word to be more feature rich and a better app.
NeoOffice isn't mentioned. After Pages it's my 2nd favourite word mangler.
If you want to concentrate in the first instance upon writing and organization, not how things will look, nothing beats a (La)TeX system. You use mark-up when writing. Then when you process the source with the TeX system, you get gorgeous output. And you can have considerable control over appearance. Ultimate output can be, in among other forms, PDF.
For scientific writing and publishing, nothing can (La)TeX for typesetting formulas correctly. But the range of document types for which LaTeX produces excellent results is much wider than that, including, e.g., parallel-columns with two languages, including many non-European ones.
You can get such a system for free: see http://www.tug.org/mactex/2011/.
(Note: the 2012 release is just a few weeks away.)
LaTeX is a typesetting-system which I love to use, but I do not consider it a word processor in that sense. However, it works great for making scientific reports and professional-looking papers.