I think I would have to say DMD BUT only because that is what I have the played the most and have the most of.  In saying that though, I love my Incredible Hulk and Jacks Open machine.  I didn't really have the opportunity to play much pinball when I was growing up so my knowledge and experience is fairly limited.

To me SS are the classic machines and I spose what I remember the most as a kid.  I would love a few more SS machines.

I bought my first EM last year and $$$$ and room permitting, I would like another one or two - I struggle to find the right word but there is a certain romance or nostalgic beauty about a nice EM machine that fascinates me.  I spose they hold a special place cause that is where it all started on a big scale and from there they just progressed as technology progressed - does that mean they are a lesser or better game, I don't think so, just different.  As Nino mentioned in his original post, there is a special aroma (it's better than just a smell) that comes out of my Jacks Open machine when you open it up.  The way they work intrigues me.  It is very cool actually seeing things moving and whirring around as the machine is played whereas a SS or DMD everything is happening on a PCB.

Really hard for me to compare and nominate one style of machine - end of the day, they are pinballs and are all cool.