Lately I've been addicted to the flash game Bloons Tower Defense 5. I think that this is the greatest tower defense game there is out there. There is a great variety of towers, great experience and upgrading system, great maps, and game types. Ninja Kiwi keep the game fresh by adding daily challenges to earn cash so that you can buy special agents that have special abilities to help you beat levels. The daily challenges restrict you to certain towers, and throw harder bloons at you to defeat, making it challenging and fun. There are so many different strategies that you can take with each map on different difficulties that the play time is endless as you constantly want to beat your highest wave. The have sandbox mode so that you can test out your strategies, and it lets you choose what bloons come at your towers to test their strengths.
My favorite flash game genre is tower defense, so I've played a lot of different ones, and Bloons TD5 is definitely my favorite of all time.
You can play this great game over at Ninja Kiwi's website HERE