I have SmartGo, GinseiIgo Hd, Champ Go HD, Igowin HD on my iPad 3.
If you want to get best difficulty on iOS with offline engine dont bother with SmartGo.
Personally GinseiIgo is giving me most problems(highest level settings 19x19)when i have enough time to focus and want to win. So my vote is for this program. You can check info about it under SilverStar name on internet.
Second best is ChampGo( crazystones).
Regardless of playing power i like iGowinHD most, due to playing style. I dont know why but maybe others with more than 1 go playing sw have their own preferenves too..
Maybe someone with enough comp go theory knowledge can explain those feelings about different playing styles between Mtcs engines.