The rankings here are much less important than the selection:
Visit it to find new apps you might like.
Unfortunately nobody is doing anything important with calendars -- because there's no iPhone API for calendars.
Update 1/2/09: Probably my most valuable app is GrandDialer -- an app that speeds use of Google's GrandCentral on the iPhone. Only of use to those with GrandCentral accounts -- but a superb value.