If an application has various parts distinctively separated into multiple containers, and each container microservice has different scaling and business variation needs, the Azure Container Service (ACS) might be a choice for this scenario. Also, if you are not yet there, but planning to implement the architecture with various containers of varying needs, again ACS is a good choice.

If an application architecture is strongly based on Docker Linux containers, ACS seems to be good choice, better than ASF.

If an application or system has a number of long running, high available stateful processes, the Azure Service Fabric (ASF) is a good choice. ASF supports the gateway pattern--so, many services are behind the gateway and transparent to the client, smoothly implementing high availability, load balancing, and scalability to a massive scale on ASF clusters.

ASF directly supports the Actor model, which is ideal for IoT applications where millions of devices are sending messages to services on ASF with a seamless integration into the Azure notification hub and Azure event hub.

If you have a serverless architecture at hand, and all the communication is event based, you want a worry free infrastructure with minimal DevOps processes, and your microservices cut down to an extremely granular level with seamless support for automatic scalability, Azure Functions are the one you should be looking for. This comes with the added advantage of cost savings on the infrastructure, admin tasks, and scalability areas.