If you lump together Squaresoft and Enix as one company, then I think it/they easily take the top spot. It's not even close. But if you count them as 2 (or even 3) different companies, then I'd say Capcom takes the top spot. I'd put both Squaresoft and Enix, individually, as competitive with Capcom before they merged though. Don't forget that in Japan, Enix was the most important third party for the whole 20th century.
curl-6 bet me that PS5 + X|S sales would reach 56m before year end 2023 and he was right.
My Bet With curl-6
My Threads:
Master Thread, Game of the Year/Decade
Switch Will Be #1 All Time
Zelda Will Outsell Mario (Achieved)
How Much Will MH Rise sell?
My Bet With Metallox