Everyone has probably heard of these, but when I first used these Apps I was blown away ... OmniFocus 2, ProCam, FiLMIC Pro, Standard Notes (Premium - it has great extensions & features and its open source e2e), Apollo, Cryptomator, Signal (Love it for the fact its FOSS and e2e), CyMath (it completes complicated math equations and breaks down the equation step by step and explains the reason for each step, just amazing), IFTTT, TextGrabber (blew me away the first time I used it), Debit & Credit (simple but really effective money management tool), HeartWatch. There are many more but off the top of my head these are some.