Google docs with Paperpile citation software. Paperpile integrates into google drive so you can cite in-line by searching title, PMID, etc. Google docs allows real time collaboration, and a history of changes that can save your life if something goes wrong. Plus it’s the cloud, so it’s not prone to issues with not saving a document.
Edit: Paperpile also generates a reference list based on your citations. You just pick the journal from their list and boom — reformatted in 2 seconds.