Star Wars on the original Game Boy (oh man, that is an impossible game! I never made it past the Death Star).
For a little bit though I was growing a bit bored with the game. The only thing keeping me coming back to it was that I needed to upgrade troops/buildings/etc and participate in Clan Wars. I could see myself slipping away from the game eventually.
But Supercell, the company that makes the game, released a pretty major update about a month ago. I won't go into full detail, but the biggest change was to add a daily 5-star bonus system. The gist of it is that once a day you can collect bonus loot that gets added to a protected treasury.
Most mobile games have a daily check-in bonus to keep you invested in the game, but I think this is the best implementation of it that I've ever seen. In most other games, you simply log in once a day and collect a bonus. In Clash of Clans, you can earn 5 stars every 24 hours (and the 24 hour clock starts once you collect your first star - so if you time it right, you could start earning stars again immediately after your 5th star).
The genius part about this system is that it gets people attacking other clans outside of war more often by creating a bigger incentive. Where before I might have attacked once or twice a day, I will attack 3-4 times in order to collect bonus stars. The update also balanced the game a little more, so while you get raided more often - you don't lose nearly as much loot in comparison because you can gain so much more.
All that said - everything they put into this update has breathed some new life into the game for me, and is keeping me interested in it. The update also showed some thought and care put into the game, and wasn't just a lazy code addition. A job well done!