The Creeps! is the one I keep going back to. The GeoDefense games are pretty good, but sometimes I feel they are unfair rather than challenging. 7 cities and TowerMadness are ok; they just don't have the same appeal as the aforementioned games.
navy patrol coastal defense is the best i have played as far as gameplay.... geodefense is by far the most difficult (but awesome!)
I third that. Ive only played the demo, going to get full version in a few minutes, and i already see that. And i used to play warcraft 3 TD's hardcore, for years. This beats anything out of Warcraft 3. Thanks.