Dragons. They may fill nearly all roles.
They covers all alignment. They may be friends of foes. They may be wise, they may be brutes. They may be cute little pets (wyrmlings, pseudodragons, faerie dragon), or they may be gigantic creatures the size of a town and the age of a mountain (or the reverse). They may be the ultimate BBEG behind all the other villains of the campaign, or just a local feature. They may even in the ancestry of the PCs (even all of them, whatever their race!). They wield magic. They even are the embodiment of magic.
And, foremost, they are the "most real" monsters ever. The dragon is a symbol present in each and every human culture. They are always present, as a force of creation, destruction, or both (see the symbolisms of the ourobouros, the dragon or serpent biting its own tail).
You can't go wrong with dragons.
Second after the dragons, the sphinx. There again, a "real" mythical creature (by opposition to illithids or umber hulk), and a frequent symbol, though less universal than the dragon (shedu, lamassu, kerubs, are just variants of the sphinx).
(I also like Titans, Naga Queens, Medusa Queens, and Sinister Harpies. And Master Gremlins. I played Heroes of M&M too much recently.)