Zotero RDF, Bibliontology RDF
MODS (Metadata Object Description Schema)
BibTeX (rich format but lacks of standardization)
CSL JSON
Endnote XML (this is actually very similar to RIS in importing from Endnote, but may have some small advantages and is one of the few styles that will preserve italics across imports.)
MAB2, MARC, MARCXML
MEDLINE/nbib
OVID Tagged
PubMed XML
RIS (this can be convenient for quick edits between export & import because of its simple structure)
RefWorks Tagged (recommend for ReferenceWorks)
Web of Science Tagged
Refer/BibIX (discourage if any other option present)
XML ContextObject
Unqualified Dublin Core RDF