30 Days of Doctor Who: Question 3: Favourite villain/alien/baddie of the week.
That's three questions. Okay...
Favourite villain - Mr. Finch, played by Anthony Head, in "School Reunion" (2x03). He has tough competition from Margaret the Slitheen played by Annette Badland (is that a real name?), in "Boom Town" (1x11).

Favourite alien - The Nanogenes and their gas-mask zombies, in "The Empty Child" and "The Doctor Dances" (1x09 and 1x10).
Favourite baddy of the week - The Reapers, in "Father's Day" (1x08).
Given the format of the question, I am always tempted to add "least favourite villain" and so on... and hey, why not.
Least Favourite villain - Davros, played by Julian Beach, in "Journey's End" (4x13). What a disappointment.
Least Favourite alien - The Gelth, in "The Unquiet Dead" (1x03).
Least Favourite baddy of the week - Professor Lazarus, played by Mark Gatiss, in "The Lazarus Experiment" (3x06).