I've been slowly going through the classic Dr. Who series. I'm just starting the Third Doctor (1970-1974 and finally in colour).
I finished it up a while back.
It's hard "watching" all the 2nd Doctor's stuff, huh?
Most of the episodes are either missing (so you're basically LISTENING to them) or are such poor quality you don't really 'watch' it as well.
At least by the 4th Doctor and up it's all color, all good SD, all the time.
Don't forget about the multi-doctor cross-overs and the TV Movie with the 8th Doctor.