End of the World was, indeed, my second favorite story of Season One. It was very, very inspired by Douglas Adams era of Who, with the dialogue alone. And, yeah, it gut punches you in the end.

You also may want to look for something called "Children In Need" special. It's a small, seven-to-ten minute "in-between" prelude that starts up two seconds after Season One, and ends about two seconds before "The Christmas Invasion". It's the Doctor getting used to his new body, and Rose trying to grasp what's happened to him. It's not essential, but, it's fun to watch...