I didn't have much time to write my response earlier.
The Forza Horizon series is by far my most played series of games over the last 5 years. Every year they come out, I am absorbed for about 2 months. The driving is a great balance of arcade and sim, the challenges are fun, and there are tons of great cars to upgrade. It just makes driving so FUN. No other games even try to do this. For this reason, to me, Playground Games is the best open world developer.