OpenOffice 3 (the unofficial PPC version), because it has better MS Office compatibility than NeoOffice. When I save a neat 1-page document with NeoOffice it often appears with the last line or two on a second page in MS Office, which is not acceptable. OpenOffice 3 doesn't have this problem.
I rarely need the other components of the office suites, but when I do my first choice is NeoOffice, since it's faster than OpenOffice. OpenOffice 3 is unbelievably slow. NeoOffice is pretty slow, too, but not like OpenOffice. At first I thought it was just because I was using an unofficial PPC port, but I've run it on Intel Macs, too, and it was just as slow.
When I do not need MS Office compatibility, I use TextEdit or Bean.
When it comes to viewing Word files, I use Quick Look in the Finder. It's nice and fast and handles formatting almost perfectly. Makes me wonder: if Apple can read .doc files so well, why don't they make TextEdit a little better?