You should check the following:
- Apple Pages for the best all-around OS X experience, and all you need is basic capabilities of easily producting beautiful-looking documents. Pages has good page layout capabilities, but if you need good handling of footnotes or cross-references, then forget it. It is free for use for those who buy a new Mac.
- Microsoft Word if you need the best compatibility with Word files, or if you need the features only Word offers (and Word offers lots of features). You can get a Microsoft Office license or even an Office 365 subscription cheap if you are a student. And a new version of Word will likely be released next year.
- LibreOffice Writer if you need more features than Pages, or better compatibility with Word, and you don't want to spend. Optimization for OS X is not very good, though. LibreOffice is free and open source. Apache OpenOffice and NeoOffice are similar and derive from the same code.
- Nisus Writer Pro if you need more features than Apple Pages while keeping a good OS X experience. If you don't need anything much sophisticated, you may get along well with Nisus Writer Express, which is a lighter version.
- Mellel is a powerful word processor designed for long and complex documents, very useful for academic and technical writing. It has an impressive set of features and is fast and reliable, but it doesn't have good Word compatibility and has a learning curve.
- Scrivener has a different approach and is useful for research and producing non-linear documents. It can export well to Microsoft Word.
- Ulysses has a good writing environment and some Word compatibility, and is particularly useful for simple writing. Very elegant interface.
- iA Writer is for just writing, without caring about formatting or anything else (you can't even change the font). It's called distraction-free writing, which is another form of saying the software has nearly no features at all.
- Mariner Write is old-fashioned and has not evolved a lot, but offers some set of features.
- Bean is free and is not being actively developed anymore, but if you have simple needs it may meet your expectations.
- Google Docs is web-based an work on any device. Not many features, but you can store and open your documents in the cloud.
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As Skaertus stated, it really depends on what you want to do. Outside the box, for research, check out Scrivener. For creative focus, check out mark down editors, like, say, OmWriter.
Word Processors, like M$Word, are really about editing and formatting. I never liked Word as much as WordPerfect. But you can't get that on a Mac!
WordPerfect has a very different approach to writing than Microsoft Word and nearly all word processors nowadays. However, Corel is not developing it very much and it has become stuck in time.
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Mellel is a rock-solid alternative (been in development for 13 years) that also has decent Word export capabilities and RTL support.
Mellel is impressive for its features. However, Word compatibility is not the best.