This is a tricky question... you will get different answers depending on who replies. Q owners are a diversifed group, not like blackberry owners who are mostly business users. You have your techies, your business users, your textaholics, your game/emulator fans, your people who just like to have cool things, your mp3 lovers, teenie boppers, 25 yr olds, 45 yr olds, etc... each one of these people will probably give you a different app when asked this question.
If I absolutley had to give one app that reigns over the others as far as my day to day goes, it would have to be "Live Search". Its just a very good, well thought out, useful application. But, I am a 30yr old educator. I'm sure the average 17 yr old would not agree.
