I saw these and thought they looked interesting. I will have to check them out. Am I correct in that there are five Bloons games? BTW, thanks to everyone's feedback I am finally familiar with and enjoying this genre! Thank you!
First TD I played was TapDefense by Tapjoy. It was a great introduction to the genre. Later I bought The Creeps and GeoDefense. GeoDefense still remains one of my favorites. Field Runners is the best in my opinion. Even to this day it holds its charm. That's my two cents.