9. Hugo Weaving
The Australian/English actor has previous form as a villain, and while his Red Skull in Captain America: The First Avenger was not his finest screen moment, there was enough in that performance, as well as his infamous performance as Agent Smith in the Matrix trilogy to suggest some prowess with similar roles. In order to reach his undoubted potential as a villain - and in order to push The Master for the crown as the most memorable - The Valeyard will inevitably require a certain level of theatricality, without wandering too far into the realms of pantomime. Playing Red Skull should have taught Weaving the value of restraint, though there's no certainty that the actor's best wouldn't come with an unwanted side-order of over-blown grandeur. Weaving also has the poise and presence to add to his menace, which would undoubtedly add another level of menace to the character (the same way John De Lancie's slightly haughty performance massively boosted the impact of Q in Star Trek.) He would be a villain to take notice of, to say the least.