For that particular purpose, I recommend UptimeRobot. It is a simple and affordable service and does exactly what you need. It also comes with status pages which makes it a really good deal.

Site 24x7 can also be used for the same purpose, which has basically the same features as UptimeRobot. If you like the UI and the features better - go with that one.

Datadog in turn is a complete telemetry platform and was designed for much more versatile application and infrastructre monitoring. Its pricing - according to that - is much higher. We've been using both Datadog and UptimeRobot, and for the use case you mentioned UptimeRobot is our choice.