Which company made the biggest impact in the gaming industry?

It's definitely not Microsoft. Even counting PC they were by no means the first, and I'm doubtful they had much of a hand when PC gaming consolidated (I could be wrong here though, so if anyone has some good resources, please share! I've only recently gotten interested in learning about video game history, and older PC gaming is an area I'm not very familiar with).

I'm inclined to go with Sega over Atari. They were much more competitive in Japan, and they were ahead of the curve on plenty of innovations like online play, even if the ideas didn't catch on until they were implemented by others. Atari was important in its early years, but also fucked the US market up so much that Japanese publishers dominated the home console market for years.