Definitely FreeOffice!
Word, Excel, and PowerPoint are de-facto standard worldwide, but Microsoft Office is very expensive.
A good free office suite should be able to open all Microsoft Office formats almost faithfully, but have you ever tried that out with LibreOffice, OpenOffice, Kingsoft, Google Docs?? I have, it's a desaster. You can open most files at least, but the majority of those will have destroyed formatting.
FreeOffice is the only office suite that offers seamless interoperability, it opened everything so far from my Microsoft Office files, and displayed them without losses - great. Same in the other direction, when I save a document e.g. as doc and open it with Word, it looks exactly the same.
Plus, it's the fastest software of all, has a nice UI, all features I need, and a perfect reliability - no annoying oddities.