I'm not going to choose a publisher, since I think that's cheating. For developer/publishers, I mean ONLY their developed games (and games in series they created that they outsourced for other titles).
It's honestly not Lovedelic. Sega has the biggest fall of any, and I would stand the most to gain even though I'm not a superfan of Sega the arcade/console/handheld developer. I adore Irem and Konami, but they largely did things BETTER than the competition, not new things.
It's S.N.K. Everything that they did they did more interestingly than the competition. Name a genre. I'd likely rather play S.N.K.'s version than the consensus favorite. It would be fun to resurrect Lovedelic or glory days greats, but I don't think they'd develop the industry bit-by-bit while still putting out substantial games. S.N.K. is my ideal mid-tier developer, and they largely followed the mid-tier developer path (read: died last generation). That the sum of their output now is 1 The King Of Fighters every 2 years ravages my interest in video games. (If I included S.N.K.-published, we're talking an outright game changer. A.D.K. and Nazca are SO underrated.)