7. Grace Holloway
Grace Holloway was The Eight Doctor's companion for the TV movie that was to act as a backdoor pilot to a new series. While the film did not lead to a revival, Grace was a cool companion and was actually first ever companion to kiss The Doctor! Not only that, but in trying to save The Seventh Doctor, she inadvertently caused him to regenerate into his eighth incarnation. As far as companions go, it was quite a unique set up.
Unfortunately Grace has only returned in comics and a few novels. While her time with The Doctor was short, she made a good impression and she would've made for a great companion for The Doctor had the film succeeded in launching a new series of the show. Being a doctor herself, she certainly would've been a capable companion to say the least.
While there may not be loose ends to tie up with Grace, bringing her back for an adventure would be a nice way to pay homage to the Doctor Who film, and could maybe even be used to iron out some of inconsistencies with the lore that the film introduced. Being one of the only American companions also provides The Doctor with an excuse to visit a few more American landmarks. Grand Canyon adventure anyone?