According to Touch Arcade the Ipod Touch 2G is the fastest.
Read the article below for more details:
Ever seem like an App Store game runs faster (or slower) on your friend's iPhone or iPod touch as compared to your own? We have seen mention of this phenomenon and it always puzzled us. Internally, as far as processor core and graphics chipset, it's all the same...right?
As it turns out, while all devices utilize basically the same hardware, there are differences that have a visible impact on games and apps that push the hardware between the various models of the iPhone and iPod touch.
Touch Arcade recently posted details of a dialog with veteran mobile developer Handheld Games Corp that reveals the second-generation iPod touch to have a distinct gaming advantage over the iPhone or previous iPod touch unit. According to CEO Thomas Fessler, the 2G iPod touch uses a v4.0 version of the ARM1176 core running at 532MHz while all other members of the iPhone / iPod touch family utilize a v1.0 unit running at 412MHz. This performance difference has a clear impact on iPhone developers and the techniques they must apply to games that push the hardware.
Our first step to increase fps performance was to introduce hardware dependent levels of detail. Where we can easily display two 1500 polygon tennis players with 32 bones each on the iPod touch 2G and maintain fast and fluid game play, the original iPod touch just chokes, and in some instances so do the iPhones. To speed up the touch, we reduced the players to 800 polygons in farther away moments of gameplay, and are now using 1000 polygon models for close ups, bringing the original iPod touch game play performance level close to that of the iPhone 3G. We’ve taken this approach across the board with great results.
Fessler indicates that the 2G iPod touch is the fastest member of the family "by far," and that the overall breakdown in performance (from fastest to slowest) is as follows.
1. iPod touch 2nd Generation
2. iPhone 3G
3. iPhone (original)
4. iPod Touch 1st Generation
Fessler suspects that the GPU of the 2G iPod touch may also be tweaked, but there's no solid evidence of that at the moment. Fessler recommends that those interested in iPhone platform gaming avoid buying a used first-generation iPod touch.