Emeryville, CA
A challenging game to me is the following and in the defensive camp:
One that keeps you off balance and on your toes. Make risky shots lucrative enough to go for, or necessary to reach an major goal / feature (multiball, etc).
One that doesn't feed every shot feed back to the flippers - force the player to make decisions and figure out how to get the ball under control.
Contains randomness that impacts gameplay, be it from pops, magnets or something else. (pops that always trickle out or feed the same way don't count)
Iron man is good example most of the above. Sure you can ramp out, but the real points come from reaching Do or Die and and to get to that point, you must hit multiple different magnets as well as super risky drone standups.
I'm also a pretty big fan of punishment, especially in a home setting - no extra balls, sensitive tilt, monster outlanes, nice and steep, no ball save. It's possible to make any game "challenging" with the right settings.
