While not a traditional IDE, Merlin [1] combined with vim/emacs/vscode is really good (autocompletion, jump to definition, type lookup and more). Merlin also gained support for language server protocol recently so that could be an option for any editor/IDE that has a language server client.

[1] https://github.com/ocaml/merlin.git

EDIT: I'd also like to point out OCamlformat (https://github.com/ocaml-ppx/ocamlformat). It works really well in my experience and makes it really easy to perform automatic code formatting, and can be used in CI to check that changed conform to the formatting style a project prefers.

Besides emacs (mentioned in another comment), VSCode with a language plugin/server like [1] works reasonably well for OCaml.

[1] https://github.com/reasonml-editor/vscode-reasonml

The VS Code extension is really pleasant and pretty much requires no setup other than installing merlin with opam.

Merlin for vim/emacs/sublime/others

Emacs?