Check out my site https://IndieHackers.com and sort by highest revenue. The "simplest" apps on there (from a technical perspective) making the most money are probably Semantria ($150k/mo), ConferenceBadge.com ($45k/mo), Nomad List ($33k/mo), Storemapper ($21k/mo), and FormCraft ($13.4k/mo).
Of course, if the app is simple to build and makes a ton of money, that invariably means that the other components (marketing, sales, distribution, etc) are tougher to crack.
Of course, if the app is simple to build and makes a ton of money, that invariably means that the other components (marketing, sales, distribution, etc) are tougher to crack.