Hired the most people.
Installed on the most computers.
Used by the most people.
Bought out the most small companies.
Nope. Microsoft isn't even the largest employer in Redmond, WA. That's Boeing. And if you only count software companies in Redmond, they still come in second. Behind Nintendo.
Installed on the most computers.
Through marketing practices such as dumping and per-cpu licenses...something that most decent people discount as dishonest at best and criminal at worst.
Bought out the most small companies.
I'm pretty sure that IBM has Microsoft beat in that category. But if you want to talk about the companies that Microsoft drove into bankrupcy and then bought them, you'd probably be right.