Train Sim World 2

Comes naturally when playing the game for the other achievements (drive 10k miles, 100 services, 100 gold medals, etc.) but if you are looking to get this one quickly, I recommend doing the London Underground services. Per full service, you will be underway approx. 40 minutes, but due to the large amount of stops, you can easily get ~500AP per stop. Each service will yield around ~20k AP.

If you are looking for a longer scenario, try the Ice and Snow scenario on the Sand Patch Grade. As far as I have played, this is the only scenario or service that gives you 97AP for driving under the speed limit (normally this is 30AP). You will end this scenario with more than 20k AP.