I think it depends on the generation. I never operated, only play, and here's my opinion:

EMs: Gottlieb was the undeniable king. Ed Krynski designs coupled with Gordon Morrison artwork made for killer games.

Early Solid State: Bally ruled this era. Their board set, daring designs, and rather complex rules really made them stand out. Sure, Williams had Firepower, and Gottlieb had Black Hole, but do either of them really compare to games like Centaur?

Later Solid State: Williams 100%. And then when Williams bought Bally and merged design teams, they got even better. From Space Shuttle onward, they had fantastic ideas that were executed so well.