if you just want to "game hack" etc go for:
Source games as the sdk is fully available
crysis games as you only need 1 offset for your entire cheat (lawl) SSystemGlobalEnvironment and you got the cryengine sdk to work with
MW2, MW3 as there's a lot of info.
BUT
Learn the basic hallo world shit first or you will be swallowed up by basic stuff like functions, classes, pointers, enums, data types etc etc.
I very much doubt you will ever make you own c++ cheat from scratch, prove me wrong