Brian Blessed: Caesar Augustus, first Emperor of Rome and founder of the Roman Empire

As you can see from the photo above, Brian Blessed is a monster performer. Check his genre credential highlights:
Flash Gordon (1980)
High Road To China (1983)
Kenneth Branaugh's Henry V (1989)
Robin Hood: Prince of Thieves (1991)
Star Wars: Episode I - The Phantom Menace (1999)
Mind you, these are just the highlights. Blessed is down for his crown, no questions asked. 

Siån Phillips: Livia Drusilla, wife of the Roman emperor Caesar Augustus

Sian Phillips was married to Peter O'Toole for twenty years. This bit of trivia only serves to validate my perception of this performer as a powerhouse. Furthermore, that most who perform at above average levels, as both Ms. Phillips and Mr. O'Toole have oft demonstrated, are driven by something akin to, if not precisely, mental illness or personality disorder. More power to them! It is this grade of performer that can really sell this genre stuff. You may or may not recognize Ms. Phillips from genre classics:
Clash Of the Titans (1981)
Dune (1984)
Ewoks: The Battle For Endor (1985)
Notice her piercing, unflinching gaze. 

George Baker: Tiberius Julius Caesar Augustus, second emperor of Rome

According to some website, George Baker "had been Ian Fleming's first choice to play his debonair secret agent, James Bond."
Eventually, he took consolation roles in The Spy Who Loved Me and On Her Majesty's Secret Service. His face may not be so recognizable to modern American audiences, but he has a great place in genre television portraying a Number Two in 1967's spy-fi classic 'The Prisoner'.

Sir Derek Jacobi: Tiberius Claudius Caesar Augustus Germanicus, fourth emperor of Rome

Sir Derek Jacobi has been in a ton of shit.

You will most likely recognize him from:
1982's The Secret Of NIMH in which he voiced Nicodemus.
1991's Dead Again with Kenneth Branagh in which he reprises Claudius' trademark stutter
2000's Gladiator in which he portrays Senator Gracchus
And if you are the art\foreign film type you'll recognize him from
1998's Love Is The Devil as British painter Francis Bacon who, as the film portrays, likes to have fags stubbed out on his bare back.
Sir Derek is a master thespian and a fine English gentleman, make no mistake, an absolute thrill to watch him perform. Swoon!