I suppose "best" usually means the latest & greatest, which would probably be Office 2010 (I'm pretty sure their latest version is Office 2010).
Generally, though, it really depends on what functions you need. Office 2000 worked fine back in the day and I bet if you only need basic functions that it would work perfectly for you.
Another option is to check out http://www.openoffice.org/
, it's a free alternative to MS Office. Back in the day no other office productivity suite could really come close to comparing to MS Office, but I think OpenOffice does a really nice job. I'm not sure if it's 100% compatible with all Microsoft documents, though. I think there were some issues with the newer docx format from MS, but that might be sorted out by now.