I've been trying to decide what my favorite era of pinball is for a little while now, and it's not easy. It's probably either the 70s, 80s, or 90s, but I love modern games and I love the few pre-70s games I've played as well. The oldest game I've played is Gottlieb's Knock Out from 1950, and I had a blast with it. I'd love to try some pre-war games soon.
EMs are great because of the challenge, immediate replayability, and unique layouts. Furthermore, I love seeing the origins of now-standard concepts in pinball. Things like the first game to have a bonus multiplier (or a bonus at all for that matter), or a spinner or captive ball that could be lit for a higher value. Things that are completely pedestrian on modern games, but were once brand-new features. The level of creativity and complexity on display in games that don't have a computer on board is fascinating.
Then the upgrade that SS bought was spectacular. Look at games like Black Knight, Centaur and Haunted House. Imagine games that intricate just a decade earlier, with scoring features that complex and varied, all made possible by that (now minuscule) computer running the show.
Each stage of pinball's history is fun to me because the primary rule has never changed: Score points. While video games have gone all over the place since their beginnings, in good and not-so-good ways, every era in pinball satisfies the same competitive feelings of beating your best record or challenging friends, just through vastly different layouts, rules and strategies.