For me, Apple's own iWork for iPad suite consisting of Numbers (Excel), Keynote(PowerPoint) and Pages(Word) works the best. They can be bought individually and support Office 2007/10's xlsx, docx etc formats without any serious issues. The cost at $10 per app might be the only problem if cash is an issue.
Pages + Numbers + Keynote = way better and worth the money. The other products don't come near. I use Keynote as standard for all my presentations now. I prep my presentations in Powerpoint on my laptop and then migrate them to my iPad for final touches and presentation. No need to carry laptop around to meetings.