[gardenerextraordinaire]

Rose Tyler gets my vote, though I do admit to having seen only a smattering of episodes from the original run.

I see you're not included one-time companions -- at least for the 2005-to-present incarnation (didn't check the others) -- and that's understandable. I am interested to see '06 Christmas Special's Donna Noble again as that character is to return regularly next season from what I have read; I believe she has the potential to be even better than Rose ultimately.