Hemorrhagic fevers (including Ebola), for example, are incredibly deadly but that really limits their ability to spread. You still have to be diligent but there's a reason why Ebola has been contained to parts of Africa the the virus seemingly spreads from animals to humans every now and again, starting another local outbreak. But it's never spread globally and it's unlikely to.
Probably the last comparable disease to Covid was the Spanish flu in terms of its ability to spread and that was over 100 years ago. It's worth noting that the Spanish flu could also spread asymptomatically, which also explains it's impact to a large degree. Pretty much every modern influenza variant is a derivative of the Spanish flu, which also tells you something about what tends to happen to novel diseases.
Y pestus is a bacteria, not a virus. We now have antibiotics that could likely combat it, which was obviously something that didn't exist in medieval Europe. Back then it wasn't at all clear that diseases spread by microorganisms at all. Plus, rodents were key to that spread and that's a far smaller problem now.
The nightmare scenario is probably some deadly variant of measles because measles is so incredibly contagious, probably more than any other disease I can think of. It can survive airborne for a very long time and in the modern era we've traced outbreaks of tens of thousands of cases to one single person. But the ability of measles to survive outside a host is also (luckily) incredibly rare.