I'd say with GCP, the managed services are key. Pub/Sub, Global Load Balancing, Firebase, BigQuery, Container Engine, etc. They just work, no need to mess with things. Price to Performance and simplicity is also good, but that's secondary IMO. I'm obviously biased, but hey!

With AWS, the ecosystem is massive. They also have the most features (though GCP and Azure have their own unique features and are catching up fast).

I haven't used Azure, can't really comment on it.