I will admit to having played, used, and binned hundreds of apps by now and the three which have me coming back again and again are:
Stanza - free eBook reader
Oxford Deluxe - English Dictionary and thesaurus
Mantis Bible Study
Lately, the updates to the Quick Office have made it a great app for: Spreadsheets, Text and word processing documents, and finally, users can enter Chinese/Japanese with little to no trouble. Thank you Quick Office!
Quick Office - iDevice Microsoft Office app