With my new Ipod Touch 4th gen after 1 year, it is Modern combat 2 sandstorm. My real first game would have been... Hero of Sparta.
Check my email history, seems like my first game is Zenonia when it was on sale for $3, on November 2010. Before that, I only bought non-game apps (and the always-free tap tap revenge ).
Bugdom 2, actually... that was either the second or third game I bought, I can't remember between it and X-Plane 9.
Hmmm off the top of my head it was either Fieldrunners or Stick Wars. (on my iPod ATM, will update once I check my iTunes account.)
Moto Chaser was my first. I remember being the envy of everone in my town as I was the only one around with a touchscreen, let alone a game on it too! Now, ofcourse it's amazing if you don't have one...
I'm pretty sure mine was Moto Chaser (I'm on my iPhone so I can't view my purchase history). I also remember playing a bunch of games on my jailbroken iPhone before the app store was launched. Simple games like tetris and solitare which were free on cydia.