Assume somehow the Spanish La Gloriosa in the 1870s resulted in an offer of the crown to the Habsburgs, and no interference from other nations. Which Habsburg has the strongest claim to the long-gone Spanish Habsburg throne beside Franz Joseph himself?
Need it for my book, I'm currently thinking of Archduke Karl Ludwig, father of Franz Ferdinand, but I don't think he has the strongest claim.
If we follow the line of descent from Maria Teresa of Spain (wife of Louis XIV):
Maria Teresa>Louis, Grand Dauphin > Louis, duc de Bourgogne > Louis XV > Louise Élisabeth de France > Ferdinando of Parma > Louis of Etruria > Carlo II of Parma > Carlo III (who marries Louis XV's only legitimate male line female descendant, Louise d'Artois) > Alicia (third child, younger daughter, married to Grand Duke Ferdinando IV of Tuscany in 1868).
Of course, if you want to take it from Felipe V (uncle of Louis XV), it'd run through his youngest daughter, Maria Antonia > Vittorio Emanuele I of Savoy > Maria Teresa > Francesco V of Modena.
If you go with Carlos III, Felipe's son:
Carlos III > Maria Luisa of Naples > Franz II, Holy Roman Emperor > Franz Karl of Austria > Franz Josef (and ergo Karl Ludwig)
If you go with Carlos IV > Maria Isabel of Spain > Maria Antonietta of Sicily (mother of Ferdinando IV of Tuscany)