Apollo - Best reddit app I've tried.

Calendars 5 - So good for the month view.

GraphNCalc83 - TI-83 calculator is so much better than the stock calculator.

Voice memos - Stock app great for recording music ideas.

1Writer - Good for editing plaintext or markdown files.

Microsoft To Do - My favorite todo app. Used to be Wunderlist until they were acquired by MS.

Lose It! - Great app for calorie counting or tracking macros.