You'll be surprised, but it's Microsoft for me.
When I started out, windows(95, yes I'm old, thanks for noticing) was the shit for me and I was quite a fanboy of it.
However, when a friend of mine introduced me to Linux, I quickly changed my mind because so many small things were way better. Then, when Vista came out, I switched completely.
That, on its own, was an adventure, but that's another rant for another time(Me and my pal were the only Linux users in the town we lived in, so without online shopping, perusing the local hardware vendors was like meeting a Neolithic tribe. It's definitely rantworthy on its own, if only for nostalgia).
The more I learned about Free Software and what Corporations could do with their power, the more I came to despise the companies I used to advocate for.
Now, it's 2018 and people bitch about what Facebook, Microsoft and their equally evil buddies do. Yet, 2013, when PRISM hit the fan, they once again ran to their arms instead of fixing the problem properly. That's about when I lost the last bit of respect for people.
And now I'm sitting here playing the world's tiniest open source violin, singing "won't get fooled again" by The Who.