TerraDave
5ever, or until 2024
Well, I have a whole thread on that subject, 5e Epic Monster Updates: http://www.enworld.org/forum/showthread.php?468639-5e-EPIC-MONSTER-UPDATES
I think I currently have just over 200 monsters identified and just half of that completed. That being said I don't know how many would be considered "classic." Here is a quick list of what I would call classic high level threats, but there are probably more.
Abominations (many types)
Angels (MM)
Beholders (both 2e and 4e had some variants higher CR than 5e)
Demodands
Demon Lords (OotA)
Greater Demons (some in MM)
Greater Devils (some in MM)
Arch Devils
Demilich (MM)
Dragons (MM)
Elder Brain (Volo - but lower CR)
Golems (MM) - could add more (adamantine)
Kraken (MM)
Lich (MM)
Purpleworm (MM)
Sladdi (white, black, & lords)
Tarrasque (MM)
Titan (Empyrean? - MM)
Yuan ti Anathema (Volo - but lower CR)
Yugoloths
I agree that in fact D&D does have iconic high level threats. Many of them. If anything, the first Monster Manual was almost certainly "top heavy" given the typical party of the time.
As others have noted, their is something circular to this. For example, many of the demons and devils in the current MM could have been made higher level threats, but weren't. This tendency to level monsters down goes back to 3e.
So, what's missing from the list above? Giants. As written in 5e they could be a high level threat. Of course an adventure with giants was just released, what is the level range for that?