Great topic... there are plenty of EM gems that don't get much love... a game I just finished this year and brought to the York show - Bally's Expressway. I never played it when it came out; in fact I never knew the game existed until seeing it at a show in the early 90s. If you are patient, have good ball control skills, and understand how to build and then score the bonus kickout, you can beat this game with decent regularity... any impatience and lack of control and your ball is done and you're going to walk away frustrated. The special feature consists of making letters in sequence, and also takes skill to light and then make specials. Most folks don't spend time understanding how to beat this game; once you play it regularly, the game gets a LOT more interesting and fun.

A few games that don't get much attention from woodrail aficionados - GTBs reverse flip games
Hit N Run and GlobeTrotter from the early 50s. Getting used to the flip setup takes some time, but these two games keep you coming back for more; lots of great ways to win, and almost getting there is easy, but getting over the hump and popping games by score and special is a special thrill, and not easy at all to accomplish. Great artwork to boot on both games.

Last but not least - Gulfstream. Everyone talks about how WMS put out a slew of games during the early-mid seventies that played and/or looked the same to save money; that's fine, but if you take some time to play a well shopped Gulf that came out during this time, then you're most likely going to really enjoy it, despite it's rather simple layout and game rules. One of the first games to support DC bumpers/kickers, because of the pure speed of the game, making 3 numbers in a line to light the special kickouts can be a challenge, especially on 3 ball play. Keeping the game angled at 5-7 degrees... it's even more challenging. Plus, there's always the "fool's gold" option of lighting 4 corner numbers on the tic-tac-toe layout to pop 3 games. It was my first pinball purchase in 1990, and it still sits proud in my line-up today.