Definitely Anthony Ainley as the Master - it was a shame that Tom only had the one encounter with him - it would have been interesting to see how their interaction would have developed over time - would it have been similar to Pertwee and Delgado?
Second place - Philip Madoc as Solon - of all his appearences in Who, Philip perfectly captures this character - Peter Cushing's Frankenstein an influence perhaps?
Third Place - Tom himself as the Titular Villain Meglos - yes I know - you all hate it, but Tom is perfect as the villain and
hero - don't know whether it was Flanagan & McCulloch who thought of it, or Christopher Hamilton Bidmead...
If anyone knows let me know!
The Doctor: I tell you, Brigadier, there's nothing to worry about! The brontosarus is large and placid!
Harry Sullivan: (to the Brigadier) This the patient, sir?
The Doctor: (looking up at Harry) And stupid!
The Doctor & Harry Sullivan - Robot