You would really need to google it for up to date testing on OS 4 with the latest generation processor. That said, shredder, fritz and hiarcs are all likely going to be weak-mid level GM strength running on an idevice. If you want can kill the world champion strength, you have to stay on the PC.
There are organizations that scientifically test the strength of chess programs, but they dont mess with iphone apps. Anyone that says "strongest" for one iphone app or another without this sort of testing doesnt have a real solid scientific basis. All of that said, Hiarcs seems to have better evidence for strength than the other iphone apps: http://www.hiarcs.com/iHiarcsChess.htm Its a bad question, since the strength of hiarcs vs grandmasters and other top programs is almost certainly useless to the OP. But that is an entirely different subject.
