It would have to be able to edit and view PowerPoint presentations. I'd prefer pptx support but I can live without it.
- Keynote (not 100% compatible with powerpoint; some things are lost in the translation from Microsoft's to Apple's format)
- Documents To Go
- Office2 HD
Personally, I prefer Keynote over what I've seen of some of the others, but they all have their limitations. Keynote's just the one I have the most familiarity with.
Quickoffice is the best, with the least bugs, however it can't edit pptx files. They need to be saved in the 2003 .ppt format. Apart from that, the app is fantastic.