Those are some lofty words to live up to, but I'll nominate a few that I think are definitely 'flagship apps' in a few categories:
-- Flipboard - by far and away the standard setter for a beautiful, pleasure to read, social news magazine for the iPad.
-- Aweditorium - as the name implies, this is a just plain awesome app for discovering (and just enjoying) new music. Great UI and has helped me discover more good new music than I have via Pandora, Last.fm and others combined.
-- The Fantastic Flying Books of Mr. Morris Lessmore - spectacularly good eBook for kids, with stunning animations, tons of fun interactive elements, and a story that is all about a love affair with books and reading.
-- Squrl - super cool video aggregator that brings you all the best videos from all the top sources. Good social elements and full AirPlay video support. Between this one and Video Time Machine you won't need to watch TV for ages.
