I checked back as far as 16 pages and did not see this question so I hope it has not already been asked.
It would be easy to say Katarina and Sara Kingdom because of the short time they were with the Doctor but both had an impack before they die. They along with Hary Sullivan seems to be the most forgotten 3. Adric and Mel seem to be the most disliked. I know it is hard to judge some due to the lost episodes but as fans we should try to get to know all of the companions as well as we can.While listening to the audios I would have to say it is Ben and Polly with Ben even being more so. We must remember they were the first companions to go though a regeneration and had a major role with keeping the show going as the change from Hartnell to Troughton set in.