I'm going to Have to go with Chaos Rings and Warp Gate. very few iphone games have the scope and polish of these two. While I found Warp Gate to be a little boring and repetitive, it is a large very well put together game and the platform needs more like it.
Warpgate may have it's quirks, but its one of the most addictive and ambitious titles on the app store to date. Point goes to Warpgate.
Did N.Y.Zombies come out in April? If so then that takes my vote. The best 3D Zombie shooter PERIOD (yes better than COD: WAW: Z)
Man, so many great games this month. Tony Hawk 2, Espgaluda II, NY Zombies, Mini Squadron: Spc. Ed., LGR2010. I would probably have to give a slight edge to Espgaluda II. My reasoning is out of all the ones above, it's the one I've spent the most time playing. Here's hoping cave will port more arcade shooters to the iPhone.
I know it's just my opinion, but I thought the 2.99 spent on nyz was the biggest waste of money this month. You spin around and shoot?..
I decided between Espgaluda II and Tony Hawk's Pro Skater 2: Just for pure nostalgia, and the crazy amount of time I've played it on my iPhone: Tony Hawk's Pro Skater 2.