N.O.V.A. is definitely one of my most favorite games for the iPod Touch/ iPhone.
Crush the Castle is also another great.
Fieldrunners is an okay game. I played TapDefense more than Fieldrunners. And TapDefense is free!
Glow Hockey is a nice one.
Call of Duty: Zombies is pretty good for any COD fan. But $10 is just way too much for me.
Also, AsciiRunner is cool. ;D
EDIT: Also, Epic Pet Wars is one of the apps I spent most of my time with.