[jman069]

Bebbled along with some of the previously recommended games are my favorite.

Bebbled for anyone who has ever played the addictive bubble popper game on a win mo phone will instantly recognize this (can't think of the win mo versions name off hand though). This I think is much much better as it's very responsive (unlike others on the market) and has several different challenges and ways to play.

Did I mention it's free? It's so good I'd donate money to the developer if it were an option...

Also the developer mToy makes some challenging puzzle-esque games if you will, like shot and phit droid.

I'd also throw in that I too have purchased Robo defense and love it as well as spades by bytesequencing I believe and if you haven't tried it Plox is another tower defense type game though I haven't had much time to try that one yet, just discovered it.