Classic Who Question

So, over the course of the past couple of years, I've been attempting to watch Classic Who through from the start, minus the obviously unwatchable eps that were lost to fire or otherwise destroyed. My progress has been somewhat underwhelming, as I've only just finished the Fourth Doctor's Key to Time story-series. November's approaching fast, with various anniversary-related things such as the classic multi-doctor audio book and whatever they'll do with the anniversary ep, and I'm finding that I'm less interested in seeing every little thing in order and more interested in getting a feel for all the Doctors.

So, here's my question for those of you who know your classics. What are the key stories that you would recommend in order to accomplish that? What else must I absolutely see before moving on from Four, and what do I absolutely need to see of Five, Six, and Seven? Eight is a bit easier, obviously.