It's Rose and Donna for me. I really liked Rose's joy travelling with the Doctor. When she's jumping up and down over Apple Grass and says she loves new earth beneath her feet and new sky above her, I believe her and like her for it. It's the same with Donna, when the fortune teller asks her if she wants to know if she'll be happy and she replies 'I'm happy now, thanks'. Their love for the Doctor just enhances it all.
I like Martha too, but I think her storyline revolved more around the Doctor rather than the travelling, the same with Jack, so the other two just edge it.