My opinion is based on what I have played. Still a few great titles out there, especially some of the early Sterns, that I haven't had a chance to play yet (Star Gazer, Big Game, Seawitch, etc.), but I'm sure I'd love. In alphabetical order.
Andromeda - fantastic artwork and my favorite of the Game Plan era; love the saucer capture and multiball release; left pop near flippers keeps you on your toes and a tough, satisfying spinner shot.
Barracora - complicated and interesting scoring, resetting drop targets, and an awesome spinner to rip; what a beauty!
Embryon - big fan of the polarizing artwork and complete art package of this machine; great shots to the left and right that keep you coming back for more; love the "secret" passage under the left plastic, the flip save, and all of the captured ball shots
Fathom - incredible in-line drop targets, multiball, great spinner location, and probably the best art package to date; like playing a piece of pinball art
Flash Gordon - fast, nasty, and unforgiving; great spinner to ramp shots and fun moving all over the playfield to max out multipliers; probably my favorite single-ball SS
Frontier - great spinner, in-line drops, and incredible collection saucer placement; I'm probably in the minority as far as digging the art package and sounds, but there's just something endearing about them
Jungle Lord - multi-ball, bagatelle, and magna-save; love the upper playfield drops and the lower playfield horseshoe
Medusa - another great upper playfield drop target game; the art package is just incredible
Skateball - great art package and an amazing bonus collect system; love the mismatched outlanes, which keep you on your toes
Split Second - the table that made me appreciate widebodies. I played it along side of Embryon at PAPA and knew that I had to own both; two great machines to pair up.