I recently downloaded Organic Maps, which provides beautiful offline maps without ads. I was just stunned with the quality.
There was also a recent thread about ad-free open source Android apps: https://news.ycombinator.com/item?id=37463662 including this project from KIT: https://secuso.aifb.kit.edu/english/105.php
What would you say is your favourite iOS app that meets these qualities?