What LibreOffice format works best in MS Office?

I have found saving my libre office docs in the older ms word 97 format works best, it gives the .doc extension instead of the 2007 .docx extension. same thing with powerpoint ppt instead of the pptx

in libre office I set the options to save in that format by default.

I have ms office 2007 at work and don't seem to have a problem going back and forth with the 72/2003 formats

From being semi-active in the LibreOffice support areas, I have seen numerous references to .docx, .pptx and .xlsx (the MS Office 2007+ versions) having a few problems currently, and that for best collaboration with MS users, the original formats (.doc, .xls, .ppt) should be used, as the exporting techniques used provide greater compatibility than .docx, .etc and .odt, .etc for MS Office.

ODT support in MS Office will never be perfect, partly because Microsoft want people to use their formats, therefore tying them into their platform to get the best compatibility, and even if people don't like it, Microsoft are a business trying to make money, unlike The Document Foundation (who make LibreOffice) who are a non-profit, trying to promote open standards and supply a great Office Suite.

Disclaimer: I am a volunteer at LibreOffice, but the above text is not influenced by my time there.

For various reasons, Microsoft does not play well with others. Your best option is to save your content in a Microsoft Office format. Try saving a test document in doc, docx, other MS options available, and have someone in your group see which one works best for them.

your best option for working with libre office and MS office is to save your documents as a rich text format, i use both Ubuntu and Windows (Libreoffice and MS office) files i have created on MS office are saved as a RTF (rich text format) i can then edit them on Ubuntu and then open and edit them on Windows

There'll never play along, my suggestion is, if you have to use MS Office for work, USE IT, don't waste your time and energy, I tried once and never worked out. Play on Linux can run Office 2007 with some limitations.

If the documents are simple text documents (Single page), in LibreOffice use .doc format, and avoid anything else but arial 10. Don't use tables or images too complex.

You could try also using Google Docs or Office Web Apps for collaborative work.