The top 5 must have apps for the galaxy s6 are Dropbox. This app enables you to save all your data for free so you always have backup files in case your galaxy s6 gets lost or broken.
Then theres always good ole netflix. No one can deny netflix is the perfect entertainment app for watching movies or tv shows. There is nothing better then netflix streaming on your galaxy s6.
Last but not least would be the weather channel app. You should definitely be using a cool weather app on your galaxy s6, and the weather channel app is where its at.