Would depend on who you mean by "most historians". Hitler considered Franco, "on his side"; did he consider him a "National Socialist"?-- it's not clear; Hitler's racial ideas made for some international political difficulties in definition, as when the Japanese became "honorary Aryans". The well known historian of fascism, Ernst Nolte, considered Franco a fascist.

Marxism makes it easy-- it starts with a theory. Fascism doesn't. There's Mussolini -- he can be said to be definitionally "fascist"; but then there are a vast array of right wing movements that share some characteristics, and yet differ in others. Is Hungary's Admiral Horthy a "fascist"? Many dispute that, calling him a "right wing nationalist"-- but what exactly is the difference? What about Argentina's Juan Peron? Again, he's commonly called a fascist-- yet his politics were syndicalist, economically far to the left of Mussolini.

Franco himself defined his politics in terms of Spain's own history, but with parallels to the foreign dictators.

“Spain has its own tradition, and the majority of the modern formulas that are to be discovered in the totalitarian countries may be found already incorporated within our national past.”

Historians are forever trying to slice definitions of "fascism", because it's not as clearly defined a system as, say, communism. Nolte has a commonly used definition:

Fascism is anti-Marxism which seeks to destroy the enemy by the evolvement of a radically opposed and yet related ideology and by the use of almost identical and yet typically modified methods, always, however, within the unyielding framework of national self-assertion and autonomy.

That fits Franco fairly nicely, but others have other definitions. One aspect of Franco's rule that differs from, say, Hitler's Nazism and Mussolini's Fascism is the role of the Church-- this makes his Spain a continuity with right wing Carlist movements. Fascism that incorporates a leading role for the Church is sometimes called Falangism, after Franco's party. The historian Robert Paxton thinks that Falangism's path to power is sufficiently different from other fascists to make them somewhat sui generis.

You can follow Paxton's arguments in Franco's Spain in Comparative Perspective in which he notes aspects of Franco's rule that differ from Germany and Italy. It's a bit of splitting hairs to argue as to whether this made Franco "not Fascist"; while Paxton highlights the differences, another eminent historian of Franco, Stanley Payne sees Franco making Fascism "fit" Spain by adopting elements of other political programs

By my lights what binds them together is more important than what distinguishes them-- its what Nolte identified, that these are nationalist movements that rise in reaction to communism, appropriating some of its appeal while rejecting Marxism entirely. You'll note that Marxists have forever been amending "Socialism", the Chinese for example, call it "Socialism with Chinese characteristics"; I think it's fair to call Franco's Falangism "Fascism with Spanish Characteristics".

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