Probably start with:

    crossref (including citations)
    arxiv
    medline

then merge in:

    dblp
    CORE
    MSAG dump
    VIAF
    archive.org paper/url manifest
    semantic scholar
    oaDOI

and later:

    wikidata
    opencitations
    openlibrary

national libraries:

    http://www.dnb.de/EN/Service/DigitaleDienste/LinkedData/linkeddata_node.html