It was a tough choice for me, as I love so many of them. However, 9's easy to slip out of jacket, fairly spry figure, high endurance, and lack of easily grabbable accessories made him the best choice in my mind. Plus Rose is fucking deadly.
If I could cross companions with Doctors they never got to be with, I may have chosen Donna instead but knowing her she would get pissy with the angel and get herself killed due to her own stubbornness. So maybe Susan. Susan always did what she was told. Well.... almost always.