Mine is Google Maps. The realtime updates, the database consistency, the magnitude of the data. At any given point, Google Maps is running several process in the background to identify the best route, obstacles on the route, time of day (to switch the screen between day and night mode), where you are on the road, etc. It does this for a billion user base while running smooth on both Android and iOS makes it the most impressive software I've ever seen. What is yours?
Also, note that ChatGPT isn't on this list as while mathematically complex, it doesn't touch hard computer science problems to solve, it essentially leaves the CAP-theorem class of problems by having a cut-off date for when it last received data.