To me it depends on what you look for, easy to hack or ease of hacking (it won't fight you the whole way). I preferred ease of hacking when learning, so no hiding CE or debuggers, I like games that have a good widowed mode for multi monitors, and I really like it when they release the mouse when paused (no need for alt tabbing), being able to disable intro videos is very nice, and being able to save at any point helps. But at some point I have to like the game else I won't really hack it or play it.
One I Liked when learning was "Dishonored" (1), there was plenty to mess with, it uses the Unreal Engine (plenty of stuff to find on this), and it really had ease of hacking (plus it loaded fast so if you deal with crashes like I did when first experimenting then this is really nice).
One I'm really injoying hacking right now is
Metal Gear Solid 5 Phantom Pain, again ease of hacking and it comes with a music player, but the loading sucks a bit (first load is long).
But I know I'll generally answer questions, so just pick one you like that won't fight you too much but has some challenges, then just ask what you don't know or understand.
