Echoing everyone... I agree, The Impossible Planet/Satan Pit is great. I haven't seen too much Eccleston, but the 2 part "The Empty Child"/"The Doctor Dances" are pretty good. It's also the introduction to Captain Jack.
"Silence In the Library"/"Forest Of The Dead" are great David Tennant episodes.
I love how they have to be writing backwards in these. Moffat thought, "I want them to be chased through a library by an empty spacesuit with a skull floating in it. How can I get there?" And he MAKES IT HAPPEN.