Genesis of the Daleks - the origin of the Doctor's greatest enemies, with the best baddie in Who
Remembrance of the Daleks - Seven at his manipulative best
The Curse of Fenric - as above, turned up to eleven
Tomb of the Cybermen - same again, only with Two
Blink - the only time time travel has really been used as a core plot element, and Sally Sparrow was a blessing after a season of Martha bloody Jones
Utopia - started pretty crap, but built into brilliance, with easily the second best Master of the lot, despite only having about five minutes screen time as himself
Only two things are infinite - the universe and human stupidity, and I'm not sure about the universe: Albert Einstein.