Iron Maiden is an instant classic in my opinion.
The rules are crazy-well thought out ALREADY, and like others said above, the complexity of the modes/Mballs keeps you coming back for more.
here are some examples of why I think these rules are some of the best
-fear of the dark (spinner mode-hit purple shot, then nail orange lit spinner) if you hit the left ramp as your first shot, there is no spinner to hit on that flipper return. keith already thought of that and made the right ramp lit for a doubler, so you can combo into the right ramp, then nail the spinner for even bigger points. And when you hit this combo, you feel satisified. Why ever go for the harder lower spinner, when the top spinner is so much easier to hit? Well, Keith already thought of that and made the bottom harder spinner worth double.
-Modes are cool to start, as you build to a mini-wizard. But they are REALLY FUN to beat, as they light a hurry up to collect a "Ripley's Letter" or "Gift from the Elves", so to speak. The modes all feel so different, and are all about as hard to beat as one another. there are no easy modes to beat, like LOTR escape the ringwraiths. Getting like 6 ramps in a row on Icarus mode is soooooooo fun. The callouts get amazing.
-the multiballs have their own personality (especially since the add a ball's require different tasks). In Mummy multiball for example, you hit spinners, and pops to get the required switch hits done, and hit the captive ball to move to the next set. Kinda boring by itself. But when you add in that you can achieve 3 add-a-balls (which involves 2 ramps for the first, then 2ramps/2orbits for the 2nd, then all the damn shots in the game for the 3rd!!), you make a wildly fun and unique multiball. For sooo long, it was "hit the mystery to get add a ball, usually" for all stern games. Keith shook this stale standard, and made add a balls super unique on this game.
I can actually go on and on at how smart these rules are, and I likely will at some point, but consider this excerpt my vote for IMDN.