This is a horrible one, partly because my knowledge is so incomplete. My family didn't like Doctor Who so chances to watch it as a child just had to be taken as and when I could. It was only when I had my own home and a television that I really was able to watch it when I chose - and then after two years of that they went and cancelled it.
I have caught up on a lot of stuff, but haven't seen everything from what is now dubbed Classic Who. For example I've seen very little Troughton and Hartnell. I've seen quite a lot of Three and Four, not much of Five because I didn't really take to him, bits of Six, which I loved, even Terror of the Vervoids, virtually all of Seven and everything since.
I guess I was pretty heartbroken for the second half of season 14 - I was so gutted to lose Sarah Jane and it took me a while to tolerate Leela.
I could just go with season 19, Peter Davison's first, but there was the Master and Adric left and there were cybermen.
Even more difficult is which series is least favourite since the revival in 2005. Each season has had its fabulous moments. Maybe Season 3, but it had daleks in New York and Sontarans and the return of the Master.Season 4 was outstandingly good.
I can't think of one whole season that was unremittingly bad. All have jewels. Sometimes you have to look harder than others, but they are always there.