Ahsoka Tano because not only is she the Padawan to Anakin before his fall to the Dark Side, but she has one of the best character developments in the entire series. Adept at dual-wielding lightsabers, she's able to take on the likes of General Grievous and Ventress singlehandedly, and is extremely athletic to boot. It's high time she makes it to the big screen, whether before her eviction from the Jedi Order or her ascent into her "Fulcrum" namesake. Furthermore, introducing her as the first cinematic Grey Jedi would be fascinating.
Whatever happens, though, the role should go to someone other than Brie Larson, as I doubt she can nail Ahsoka's positive, determined attitude down right.