I think Kingdom Rush was my favorite game. I stopped playing a couple of weeks ago because I just played it to death. I did every option and achievement except for catching a (stupid) fish. I liked it because it took time to learn and play. You could beat a level different ways. The upper levels took a long time for me to beat. I had to use the cheat videos on YouTube to get an idea about how to start them. And, I still can't always beat the upper levels.
I really liked Steambirds. I didn't like "Survival" so much. It took some time to master this game. And, there were different ways to beat the enemies. I was very happy to find the Bonus Levels which you could play without going through all the easier levels.
For the past couple of weeks I have been playing Zombotron and Zombotron 2 enough times where I just can't play them anymore.. They are fun because they take time to play through the whole game. There are different options (weapons) to use. And, you don't have to follow the same path through the game.
I was really enjoying Pirate Galaxy. The problem is that the library WiFi system I use just blocked it.
I played and enjoyed the Sugar Sugar games. They were fun. (I also liked figuring out a game not on this website: 3 Slices.)
I do hope there is a Kingdom Rush 2, Steambirds 2 and a Zombotron 3. Right now I am searching for another current favorite game to play to death.