I liked the button-mash esque codes from SA and before. Phone cheats are too clunky unless there's like a speed-dial for them. More of the cooler cheats need to return too like different themes and such.
Cheats should be accessed through the pause menu, and there should be an infinite health cheat or something, and not just being able to fill it. Even a simple cheat like that could provide so much fun, and same with simply turning off the cops, as opposed to getting rid of your wanted level.
Though, after quite a few hours of playing GTA4, I remember being able to use cheats on the phone without even thinking about it. Health, for example, Up, Up, A, Down, A.