I've used CloudOn which is OK on iphone and ipad. Not sure if they have an android version. Thanks for starting the thread though - will check out some of these others.
and Quick office (came with the phone I believe). Out of the 3, I'd say Documents to Go was my favorite. I can actually do more with that version of Office and Amazon actually had it as a Free App of the day many moons ago.
If I really want to get 100% use to an office product though, I just use Remote Desktop and will remote into my work computer which has a full Office suite. The only caveat is that you have to have access to your workplace Wifi and your phone has to connect to that Wifi connection. There are other remote desktop apps that will allow you to connect to your computer via a download on your computer, but Remote Desktop is the most robust app IMO. Amazon.com: Xtralogic Remote Desktop Client: Appstore for Android
I have tried most of the ones recommend here. I use Kingsoft, it is free and I can't find anything I do on my phone with Excel or Word that it won't do. It's also a good pdf viewer.
The biggest plus for me is how well it integrates with Dropbox, Skydrive, and Google Drive.