OK - this is difficult if not impossible, but for an oldie like me my all time faves would be:

Black Hole - great sound, reverse lower playfield, mad multiball, very hard to master.
Pharaoh - the best of the Williams split playfield series in my opinion - the only game I know of where the upper playfield is larger and more important in gameplay than the lower playfield.  Check out the humour in the playfield and plastics artwork.
Paragon - was not a fan of this game back in the day, but now I have one and it is not going anywhere!  Great artwork, great gameplay and a hard game to master.

Honorable mentions to - Black Knight, Centaur, EBD, Haunted House and my beloved Sterns, Stargazer, Quicksilver, Nineball, Seawitch and Meteor.

cheers
Ian