One of the best games ever made. The game perfectly combines free-roam with story missions. I've finished the main story and there are still plenty of things to do: finding hidden locations, completing challenges, doing side missions, robbing houses/building/trains and hunting animals.
The wildlife in the game is amazing, there are over 200 species and they all behave like they would in real life.
The map is also great, the climate is diverse and there's plenty to explore. The whole map is bursting with detail and every nook and cranny has interesting infrastructure, landscapes and backstories.