The only person who knows if Fathom is worth it is you. I did not really care for Centaur and the instant that I sold it, I started looking for a Fathom.
I grew tired of EBD but it took a long time. I never did like the design of Quicksilver, Star Gazer, or Seawitch, even when they were $1500 titles.
I think Skateball, Flash Gordon and Frontier are all very good games. I would consider owning one if I sold something else.
But, a ton of classic Bally machines are all very similar. Finding the perfect game for you that compilations your collection could take years or even decades. In my opinion you CAN have too many. I typically prefer to own 2-3 of each pinball era/company and look for games I can't seem to play locally.
IMO, Harlem is the FIRST SOLID STATE pinball that can/should be considered a tournament game that you can set up extremely difficult.
On some other opinions of classic Sterns/Bally machines others listed... a lot of the other games mentioned are great but I can't justify owning most of them. Pinball takes up quite a bit of space!
One game that I think is fantastic that was not already mentioned is Mystic. There is just something great about that game.
Oh, and whomever thinks Galaxy is not a fantastic game, you are wrong.