1. AWS is the clear market leader, tons of services, good support options, but you'll need pretty expert-level ops people to make sure everything is working together securely. You may have to build more plumbing than you think. 2. Azure is great for companies that are already MS shops and it makes lots of MS tech (.Net, SQL Server) super easy to use in the cloud. 3. If I were starting a startup, I would go with GCP. IMO their services are easier to use with a smaller ops team, and they have lots of services that scale more easily as you grow (e.g. Cloud Spanner). But not as good enterprise-level support.
I see lots of excitement for GCP in the startup scene, and as these startups become more mature I think the "enterprise-level"-ness of GCP will improve.