Hyperledger Composer allows you to work with many IDEs. Two well-known ones are Atom and VS Code, which both have good extensions for working with Hyperledger Composer.

Atom lets you use the composer-atom plugin (https://github.com/hyperledger/composer-atom-plugin) for syntax highlighting of the Hyperledger Composer Modeling Language. You can download this IDE at the following link: https://atom.io/. Also, you can download VS Code at the following link: https://code.visualstudio.com/download.