In this strange complex of time and space anything can happen

I think Rose works well for people who already have a positive attitude towards the show, but for people who are a bit more undecided, I think the recent addition of The Fires of Pompeii works really well-- while it contains some references to the bigger S4 story arcs, it works very well on its own too. From the Eccleston series, The Empty Child/The Doctor Dances could work too, because we need to go through the whole "introductory" phase again when we meet Jack, but I think the problem there is that those episodes are so damn good that a new viewer might be disappointed by anything else. The Christmas Invasion and School Reunion would also be good ones if you wanted to start someone on David Tennant instead of Eccleston-- I find that people tend to fall in love with Eccleston's Doctor very easily (I know I did!) and then it takes a while for them to accept Tennant-- so in some ways it's best to start them on Tennant, then go back to Eccleston-- they'll still fall in love with him, but they won't be as wary of accepting the Tenth Doctor.