Open world games with a lot to explore and do. I still play GTA V online because it is fun to just aimlessly drive around, golf, explore, whatever. Also, as you can see by my avatar, I liked sonic adventure as I could explore when not doing the missions. They shifted away from it though and the series has lost its way IMO.
Unfortunately I feel like most people dislike open world games or developers don't like them maybe? Example: Red Faction Guerrilla was open world and it was so much fun exploring and destroying buildings. The next game they did: Red Faction Armageddon was linear, sucked, and it got a bad score. Oddly enough, Most red faction games have been added to the xbox backwards compatibility list and of course Guerrilla was left out.