Heroku is no doubt the easiest to set up and use. I find their free tier too restrictive, however, since (1) apps are put to sleep pretty aggressively and take time to wake up, resulting in failed requests and (2) they have a set quota for total free hours/month across all apps on your account.

GCP free tier provides a lot more in comparison (app engine hours, cloud storage, memcache, pub/sub, cron, egress, cloud functions and more), but you have to be willing to tolerate the vendor lock-in.