Mine is definitely TanZen. It has so many levels (like 426?), and it;s only $0.99.
I also really like Fuzzle. Out of all the apps I've bought, I've used this one the most.
Gamelab is also cool, because the promise a new game every month.
Strategery is like a sexy/good looking version of Risk. A little less fun, but nicer looking.
7 Cities and Besiegement are great TD games.
Deep Green is the best Chess game ever.
I'm pretty sure that most of these are under-rated.
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