4. Professor Yana/The Master (Derek Jacobi, Utopia)
Perhaps a little too high up on this list due to his limited screen time Derek Jacobi's presence in Doctor Who told the world two things. One, Doctor Who was truly a media juggernaut once again and two, all you need is ten minutes and you can be one of the most memorable incarnations of one of the most famous characters in with world. He goes from the utterly adorable Professor Yana, who fangirls with Ten over science, history and technology and is willing to sacrifice himself so the human race has a chance to fly to a menacing, completely unexpected figure who completely defeats the Doctor and leaves him stranded at the end of the universe. And all of that's in about ten minutes. Absolutely brilliant. Fantastic. Jacobi's performance is so strong that even as he wears the same costume, he really does seem a completely different person when he becomes the Master he walks, talks and speaks differently and can only be taken down by a complete wildcard, his assistant Chantho. Props to Tennant's performance when he discovers Yana has a fobwatch, implying he knows it must be the Master. After all, who else could it be?