A data analytics company is running a proprietary batch analytics application on AWS and wants to use a storage service which would be accessed by hundreds of EC2 instances simultaneously to append data to existing files. As a Cloud Practitioner, which AWS service would you suggest for this use-case?

For this use-case, I would suggest using Amazon Elastic File System (EFS). Amazon EFS is a scalable file storage for use with Amazon EC2 instances. It allows hundreds of EC2 instances to simultaneously access the file system, making it a good fit for the described scenario.

Here's why:

1. Shared Access: Amazon EFS is designed to provide massively parallel shared access to thousands of Amazon EC2 instances, allowing your applications to achieve high levels of aggregate throughput and IOPS with consistent low latencies.

2. Scalability: Amazon EFS is a fully managed service that is designed to scale up and down automatically as the files are added or removed, so your applications have the storage they need when they need it.

3. Data Consistency: Amazon EFS provides file operations that are atomic, consistent, isolated, and durable. This means that all file system operations are atomic, and once written, data is consistent across all access points and will be durably stored throughout its lifecycle.

4. Appending Data: Amazon EFS supports highly concurrent workloads. This means that if you have hundreds of EC2 instances that need to append data to existing files simultaneously, EFS would be able to handle this workload.

5. Integration: Amazon EFS is integrated with other AWS services, making it easier to use with the AWS ecosystem.

6. Security: Amazon EFS also offers several security features, including encryption of data at rest and in transit, integration with AWS Identity and Access Management (IAM), and detailed access logs.

So, considering the requirements of the data analytics company, Amazon EFS would be the most suitable AWS service.