Quickoffice HD is pretty good. I've been using it on an iPad 1 for about 2 1/2 years. They've added quite a few features over that time, most recently the ability to comment and track changes in Word docs.
The only downside is that the autosave feature frequently fails, especially on doc longer than a few pages. That might be due to the iPad 1's limited RAM, but I'm not certain because I'm autosaving to the cloud.