Could be very different for sited vs. home pins.

On site, the theme will be the first thing that attracts me - if there's more than one pinball of course. If the venue has just one Sega, I'd be playing it!! Given a choice, I think the theme would determine where the coins go first, and that means that its hard for a manufacturer because themes will appeal to different people. I suspect that's why Sega went down the mostly movie-themed games, at least a game at the time of the big picture release might have attraction.

At home depth might be a strong consideration. For me, I don't think it matters. AFM is relatively deep, Firepower typical 1980's no depth, yet they rank as pretty much equal favorites for me.

Thinking about why I have and prefer AFM over MM, and everything else in the collection, I think theme gets the highest part of the ranking equation for me, closely followed by a flowing game rather than a stop/start game (ie why I can't handle TSPP despite loving the humour or TZ despite its other attractions and the fact I love the theme).

Humour is important to me in later games, one of the attractions of AFM and BBB.

I also am very much partial to the artwork - another reason why Stern games loose out to me as they are too much photoshop looking.

So for me it seems to be -

Theme
Flow
Artwork
Humour.