Such a vague question. You can make an argument for almost any game.

So many things can affect longevity...rules, modes, theme, sound, the satisfaction of a certain shot/completion...

I've pretty much played em all and owned most and for me it's not about MORE. I don't enjoy the "packed" games like TZ, TSPP, or Hobbit any more than I do the simple ones, in fact I tend to prefer the latter. It's the satisfaction I get from certain parts of that game in the little time I get to play them. I suppose if I was playing them for hours every day it would be different but then I'd find something else to do with all of that time.

Things I enjoy about some of the games currently in my game room:
BK2K - Awesome music to get me in the game, just trying to get a multiball going and do anything with it.
AC/DC - Complete a few songs by making the shots that I need to.
NGG - Whack some gofers, hit a hole-in-one when I'm actually aiming for it.
TX Sector - Cool sounds and effects.

I think that, like most things in life, the key to enjoying the pinball hobby is to find what you like, play at proper intervals (don't wear it out), and mix it up every once and a while. Pretty much the same advice for a good sex life