Please excuse the flippant title. I figured it was an easily digestible way for me to initiate this discussion.
I am currently compiling a list of "must read historians" for a non-medievalist friend, and it occurred to me that other medievalists must have some opinions about the "classic" authors of the field. In other words, historians whose works have influenced the field (either of medieval history or of any measure of academia*) in some meaningful or significant way.
I'm thinking of historians like Charles Homer Haskins, Henri Pirenne, Georges Duby, Joseph Strayer, and so on.
* As it happens, medievalists have had some significant influence on greater academia, which is why I thought this question would appeal to the general /r/askhistorians community, as well as to those who are interested in the Middle Ages. For instance Marc Bloch, the French historian who was famously executed by the Nazis, helped found the Annales journal, which later developed into an ideological school.
I'd probably go with Michael Prestwich. For my field his work on both the logistics of warfare and Edward I are really quite important. He still has some of the best work on the changes to the composition and recruitment of English armies that happened over the thirteenth and fourteenth centuries.
I gather from another student at my university that he's also done some important work on English legal history but I don't know anything about that.
For early medieval religious history: Peter Brown. He's on the cusp of medieval I guess (I mean, he more or less created the field of Late Antiquity), but there's none better if you want to delve into the intellectual and religious history of the Late Ancient/Early Medieval Mediterranean world as well as the origins of some of medieval Christianity's most enduring features: the cult of the saints, monasticism, holy men, etc.
The man also knows something on the order of 26 languages-- so if a list of hall of famers is supposed to make me feel inadequate...
And of course, the Medieval History Hall of Fame would not be complete without Ernst Kantorowicz. The King's Two Bodies (1957) is the foundational work for our understanding of the political theology of medieval kingship. Not to mention the man's crazy story. Born to a German-Jewish family, Kantorowicz fled Germany in 1938 after refusing to sign a Nazi loyalty oath. He ended up teaching at UC Berkley, where he again resigned upon refusing to sign the oath of allegiance required of UC faculty during the Red Scare of the 1950's, after which he moved to the Institute for Advanced Study at Princeton.
I'm a big fan of Stephen David Crouch, even if his legacy is still in progress. Can't think of anyone who does better work on aristocracy and chivalry.
Edit: David Crouch. I'm an idiot.
Marshall Hogdson definitely deserves an inclusion, if you want to branch out into the Middle Ages outside Europe- though not a medievalist specifically, his theories of an interconnected Eurasia have been vital for an understanding of how medieval Eurasia worked. "The Venture of Islam" is still a critically important book, and forms a lot of the basis for a modern understanding of the medieval Islamic world.
Kathleen W. Hughes gets my vote. I'd argue that her work on Irish church history remains unsurpassed.
Steven Runciman. Honorable mention to Will Durant for being all around awesome.