2. Duggan (City of Death)
Honestly, when I first saw City of Death I couldn't believe that having Duggan join the TARDIS crew wasn't end game. He immediately developed a rapport with both Four and Romana, and seemed to be sort of guy who could take unusual situations in his stride. Duggan managed to be of actual use to the Doctor, and brought a wonderful sense of humor to the proceedings. I've always found that companions work best when they provide a stark contrast with the Doctor. Duggan was much more comfortable using violence to solve his problems, and I think it would have been fun to see him and the Doctor reconcile their very different methods. I can't imagine him sticking around the TARDIS for too long, but his interactions with the Doctor and Romana are easily worth a season or so.