Historically, they've recommended Heroku... however, their dynos have a cold-start problem, causing your snake to sometimes timeout. I would recommend using an AWS free-tier instance if you have the option.
I remember that, they recommended a workaround last minute for it. Our snake depended on simulations so we just spun up a beefy GCP instance for a short period at the expense of a few cents.
If you're looking for something easy to get started with digitalocean / linode offer reasonable plans ( and you can always host a site or something with the extra power you'll have spare )
Alternatively aws offers 12 months limited 'free' usage on many of its services last time I checked - you should be able to use their most basic ec2 instance within that free tier I believe
All the options other people are suggesting are good, but if you host on Google Cloud Compute's us-west1 region you'll get very low latency as that's the region from which they send the HTTP requests.
I'm a fan of Google app engine for pain free deployment of simple things. If I have a go at this later, it'll be a Phoenix (Elixir) app running on app engine flex.