Avoid Word 6, it's big time bloatware. 5.1 is the optimum version for pre-PPC Macs, Word 98 may be best for early iMacs/iBooks. I too have heard good things about 2001 but haven't used it.

Word 2007/2008 is disgusting. The interface is worse than something a three year old could design. Nothing is in a logical place and the layout is annoying and distracting. Plus the new document format is a pain for those still using 97/98/2000/any other older version.