Okay, this is a fun one. I hope some of the other medievalists will stop by and critique/add to my list.

So medieval history, the academic discipline, has got to be one of the most self-obsessed...er, self-reflective...er...out there. There is an enormous emphasis placed on "where we came from" as a field. So like other fields, we "need to know" the top people working right now or in the past 20 years on whatever more specialized topic you do (as a late medieval/Reformation historian, my list is going to look very different than /u/alriclofgar's as an Anglo-Saxonist, which will be different than /u/Yazman who studies al-Andalus, and yet we are all medievalists). BUT there's also a list of foundational figures from the late 19th/20th century that Everyone Knows. Offhand, I'd have:

Marc Bloch (Marc Bloch always goes first), R.W. Southern, Charles Homer Haskins, Henri Pirenne, Jacques LeGoff, Herbert Grundmann, Ernst Kantorowicz, Gerd Althoff, Caroline Walker Bynum, Johan Huizinga, Joseph Strayer, Eileen Power

ETA: Georges Duby

A bit peripheral to the Middle Ages now, I think we'd also be expected to recognize Peter Brown, Eamon Duffy (or at least understand the phrase "stripping of the altars"), and Fernand Braudel.

Oh, and I think we're at a point of both being supposed to know Otto Brunner and being congnizant of his Nazi sympathies/activities.

Many of these scholars, though not all of them, have a massive distinguished body of work but are best known for one or two of them that, in retrospect, drove a major paradigm shift in medieval historiography. Despite Haskins' wide-ranging interests in early and high medieval history, especially in England, his name is pretty much inseparable today from the phrase "renaissance of the twelfth-century." Ernst Kantorowicz goes with "the king's two bodies," a theory of medieval political thought. Southern said, "Hey, but isn't culture as important as politics?" Bynum was hardly the first scholar to study medieval religious women, but her perspective and argument were strong and compelling enough to basically invent a new subfield.

So what I'm saying is, it's not necessarily that you have to read everything that all of these people wrote. In fact, a lot of the individual points and arguments they made have been superseded by later scholarship. But their contributions to shaping medieval studies--that's what to know.

For my own field, armour and weapons:

• Claude Blair - His European Armour c. 1066-1700 is still the go-to general history of armour. He made a lot of terminology (always saying mail, not 'chainmail' and never 'plate mail') standard. His way of thinking about armour - in terms of its underlying construction, how it fits together - is still influential.

• Alan Williams - The Knight and the Blast Furnace was a game changer when it came out in 2000. By analyzing the metallurgy of armour he was able to quantify a lot of what had previously been guesswork. For a field that is still very closely tied to art history, he did a lot to bring in materials science. He also collects a lot of previous work on the history of armouring.

• Tobias Capwell - I suspect Capwell's best and most influential work is in front of him (he's not old!). For an analysis of armour's function, he's hard to beat.

• Ewart Oakshotte - say what you will about his famous typology of swords and all the fanboys that slaver over it, his emphasis on blade geometry and function rather than hilt decoration is really important. Even if some of his functional explanations are guesswork.

Then there's some historians that are important but haven't published much book-length works in the field. Pierre Terjanian has been an influential curator, first at the Philadelphia Museum of Art and then at the Metropolitan Museum of Art, and he has written a lot of great stuff about the armouring industry in various German cities. Silvo Leydi has written quite a bit about the armour industry in Milan.

This definitely isn't what you meant, but I'm gonna add to the great lists here by throwing in some literal medieval historians and works you should read if you want actual medieval perspective. These obviously aren't histories you take at face value, but they were big works in their time and help understand a lot of what we've learned since. I'm admittedly pretty Britain-centered, though.

• Geoffrey of Monmouth's Historia Regum Britanniae (medieval bestseller on Britain)
• Roger of Wendover and Matthew Paris's Flores Historiarum (two chroniclers for one work)
• Boethius's De Consolatione Philosophiae (foundation for medieval European philosophy)
• The Middle English Prose Brut Chronicle and its continuations (lot of different versions and continuations exist, but this was the most popular book the late-medieval English world read, so it's essential propaganda reading)
• Virgil's Aeneid (an old professor once told me you just can't be a Western literary historian without knowing the Aeneid, but mileage may vary for other historians)

As for modern historians, I'm still playing catch-up myself, but I'm seconding March Bloch. Even in a literature-focused department, he was first on the bibliography when I walked through the doors, along with Georges Duby. It's worth noting that there's kind of an ideological divide between English and French historians in the 'canon'. You'll notice that England and American put out titles likely to start with "The Reign of..." while France gives us ones with "Society" and "Rural," if that makes sense. The best I can say is browse different areas and read abstracts of articles to see what pulls you in. Sometimes you can even search for "[Subject] bibliography" and get a reading-list other scholars have put together. Better yet, find a book that grabs your interest, then check its bibliography and see where their sources came from. If a subfield has a 'canon', chances are good they'll show up a lot in references.

Norman Cantor, Inventing the Middle Ages

Marcus Bull, Thinking Medieval

Marjorie Chibnall, trans., The Ecclesiastical History of Orderic Vitalis

William of Malmesbury, Gesta Regum Anglorum, trans. R A B Mynors; Rodney M Thomson; Michael Winterbottom

Ibn Khaldun, Muqaddimah, trans. Franz Rosenthal

The classic medieval military historian has to be Charles Oman. He published his Art of War in the Middle Ages in 1885, but it's his revised two-volume edition of 1898 (Re-titled A History of the Art of War in the Middle Ages) that really cemented his reputation as the father of medieval military history.

Also quite influential, and of a similar era, is J.E. Morris. His writings on the English longbow and Crecy are still being debated, and while I disagree with most of what he has to say, I cannot say that he's not very influential.

While talking of vintage historians, it's hard for me to not mention Ralph Payne-Gallwey, whose history of the crossbow is still the most widely available book on the subject despite being over 100 years old.

For more recent historians I have to mention John Keegan (particularly his Face of Battle), as well as Kelly DeVries, Anne Curry, and Maurice Keen.