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.