I’ve always been partial to Jay Robinson as Caligula in The Robe and Demetrius and the Gladiators. He had a really creepy look, and I heard somewhere that he got so much into his character that he suffered mental problems afterwards.
On the other hand, John Hurt was (of course) fantastic as Caligula in I, Claudius. Not as over the top as Jay Robinson, and probably more believable.
And of course, there’s Joc-um-(mumbledy mumble) Phoenix, who portrayed a fairly three-dimensional Commodus in Gladiator.
Any others that rank with this trio?
BTW, has there ever been TV or movie portrayal of Elagabalus? I’d pay money to see that one!