Google Cloud IoT - Fully managed IoT services from Google | Google Cloud Platform
AWS IoT Platform - Amazon Web Services
Azure IoT Suite | Microsoft Azure
Internet of Things | IBM Cloud
There are tradeoffs between these, but what’s right for you probably depends more on you than it does on these platforms. Realistically, if you have a particular project in mind, take a look at at least these three and see if one of them feels right for your needs, and/or better than the others.