IMO, FileMagic, Docs To Go, and QuickOffice are the best available. All three have a WiFi sync with them to get Docs over to your iPhone. They all three have a good Filing System with them.
File Magic is only a "Filing Cabinet". It will file, store, and view(only) docs, pictures, movies, and music. It handles the "filing" part the best of all three.
Docs To Go can create and edit Word docs, but only view Excel sheets for now. It can also view many other doc formats. It has a "filing cabinet" also. Not as slick as FileMagic but you can drop and drag entire files VERY easily to your iPhone. It will also sync any changes that you make to those files. It probably handles this the best of all three.
QuickOffice is the most full featured of the three. It will view all kinds/formats of docs and it can create/edit Word and Excel Files. It also has a "filing cabinet". You can also drop and drag entire files via WiFi and if you are a Mobile Me user it also sync with MoMe. This is why I use QO the most!
They also just released an update that allows QuickOffice to download/file/edit email attachments. No other app does this yet. D2G is not far behind though.
These are the three that I use and can recommend. I am sure there are others as well.