Bloons Tower Defense 4 must be the most addicting tower defence game ever created! The previous Bloons Tower Defense games all took the online world by storm and with the fourth edition it gets even better.
If you haven't played the previous parts in the series I really recommend that you start with Bloons Tower Defense 1 and play your way thru Bloons Tower Defense 2 and Bloons Tower Defense 3. Every game gets a bit more advanced with extra features.