For me, it was Robots of Death. I was 5 years old and it truly scared the hell out of me! From then on I was hooked! Destiny of the Daleks is really special as well - I remember (being obviously infinitely more mature as a 7 yr old) watching it with my favourite uncle while my bro and dad went to the football. I didn't use to go to the football because I wouldn't get back in time to see Dr Who. Funny how these first memories of Who are far more than just memories of a TV show - for me it defines a large part of my childhood. If I watch the continuity announcements on some of the DVDs I get overcome with waves of nostalgia
Great OP Peri