Doom. I believe they have updates planned too so you might end up getting even more bang for your buck (someone else confirm, pretty sure Carmack mentioned it in an article).
I can't necessarily disagree with the comments that say "Doom", but I will point out that Wolfenstein is only $1.99. And for that $1.99 you get a lot of content. You get the original 3 episodes (10 levels each!), plus another 3 episodes known as the "Nocturnal Missions", PLUS the entire retail bundle of Spear of Destiny! The only thing they didn't throw in were the so-called "Lost Missions" (which are actually owned by FormGen) and the kitchen sink. Actually, I take that second part out. ID *did* include the kitchen sink. You can create your own levels to download for the game! (Though don't bother with ID's instructions. Just use WDC. It exports the iPhone MAP format directly.) So while Doom may be a better game overall, Wolfenstein is the better value. Disclaimer: I am not in any way associated with ID Software or their games. However, I do love the Wolfenstein 3D iPhone app. I've even started a website to trade maps. I've also been hacking the source code to see if I can improve it. ________ Iolite Review
