Still think Maps with Nav is the best app, most important, and most used, but I would add to my prior post to fill out my top 5 because I don't see others here heralding the boring apps I love:
* Pure Calendar Widget - Best agenda widget out there.
* DirecTV - got it thinking I would use it once a month now I use it every day and it's on my center homescreen
* Tower Raiders - no games getting any love here? Best Droid game IMO.
* Weather Widget - no frills, just exactly what I wanted. Downloaded like 15 weather widget apps and this is the one I kept.