I am a heavy Bally SS fan.
Had about 25 come and go, said I'd never sell some and have ended up selling.
Flash Gordon always remains.
That's my vote!
But you cannot go wrong with EBDLX, Centaur, Paragon, Mata Hari, Fathom, Frontier, or Skateball either.
So much excellent old school out there - it's what makes pinball great.