Jadewarlock, one of the audios I was listening to had Mel say she doesn't scream!!! I agree that Colin Baker really comes into his own on audio. I find myself really enjoying his Doctor much more then the show.
Critical, you don't need to go chronologically, but if you wanted to start with Pertwee (3) and go from there. I loved him with Jo Grant. Some of the old ones are good, but as Jadewarlock said there aren't many Troughton's left. If you want to see a very good one of his Tomb of the Cybermen is excellent. For the first Doctor, An Unearthly Child is the one that started it all.
Also, not sure how many of you watched the movie An Adventure in Space and Time about Hartnell and the creation of the show. Highly recommended. I wept at the end.