1. Alphabet/Google
2. Apple
3. Facebook
4. Match Group
Match group is lucky they're not getting much attention, but they own all major dating apps. They need a bit of smacking.
Reasons:
For Google, the biggest problem is their bundling of Chrome, YouTube, Gmail if you want to have Google certified android phone that has the Google play store. Google should be prevented from having their own WWW crawler too. The crawler should be made into it's own company and data should be purchased by Google and any other company that want to purchase it at the same costs.
For Apple, it's their app store. The app store needs to be eliminated. Apple has a problem with not allowing competing browser engines on iOS and misleading people by saying it's for privacy reason. Eliminating the control of the App store from Apple is going to fix the problem.
For Facebook, Instagram and Whatsapp should be spun off to their own entities.
For Match group, break off tinder into a separate company. And prevent any future dating app accusations by Match group or Tinder. NO MERGERS.
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From the Chinese side:
I'd like to see Alibaba group and Tencent being investigated.
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Edit: replying to calls below for travel industry.
The travel industry is already competitive enough, and the situation in these areas should be reviewed after a year or two.
For now, these are the critical problems for tech. Their control is taking the breath away from smaller companies and reducing innovation.