I've seen most of the old show and I really don't like it. I'm more of a fan of the audio series. But if I had to pick for the old series: Vengeance on Varos. And for the new, that's an insanely tough choice. There's way way too many amazing stories. Probably The Empty Child/The Doctor Dances with too many runner ups to mention but a couple really good ones: Midnight, Turn Left, Blink, The Eleventh Hour, The Beast Below, and so many more.
Yeah we can start now if you like and just keep it open in case others want to join. Just got to work out a story between us first. What are some things that everyone is interested in? Not necessarily even sci fi, we can do literally anything. And we can do multiple stories. Indeed, I was going to do a Hogwarts student as a companion once.