This is one of those keep it simple kind of posts. Smartphones are made to run apps and here are some greater starters for everyone.
There is virtually no place you cannot access information you have stored in Evernote. No seriously, you can get it on your refrigerator. It is an all in one solution for notes, reminders and lists that has rock solid performance every time you use it.
If you watch any video whatsoever, then these are a must. It also doesn't hurt that they are very well done apps.
The standard in online storage of files. The ability to have important files just a click away on any device you use is super convenient. Yes there are many competitors (Google Drive, iCloud Drive, Box, OneDrive) but I keep falling back to using Dropbox. It is one of those things that "just works." Throw those thumb drives away right now.
If you use email, please don't stay stagnant on the mail app that came with your phone. It can actually be enjoyable to use wit Mailbox. The ability to quickly go through a list of mail and get to that mythical inbox zero is phenomenal. Each message you receive can quickly be swiped to delete, archived or set to come back later. Get rid of the emails you don't need to see until you actually need to see them.
What? You're not currently addicted to some game already? Great, download Threes! and you will be. You are welcome!