Bloons TD 5
Bloons TD 5 is an amazing Tower Defense game. Before this, my favorite Tower Defense game of all time was Bloons TD 4... so this was a logical extension.
It's just as good as I hoped!
If you aren't familiar with the Tower Defense genre this is how it goes. Some kind of invading force is coming down a path and if they get to the end of the path bad things happen for you. Typically if a set number of enemies make it to the end you lose. You place towers along the path to try to stop the invaders. The invaders come in waves and between each wave you get some breathing time to build new towers or upgrade existing ones. That's Tower Defense in a nutshell.
Bloons TD is that, except the invading forces are balloons and the towers are monkeys. It's amazing.
This is a game I can easily recommend to anyone with a smart phone. Bloons TD 5 is well worth it. Or just go try it in your browser right now for free.