I'm torn between the old dear in Planet Of The Spiders ("No you shan't, I shan't, don't take him..."), the kid in the Flesh 2-parter, and the kid in Utopia (bit unfair, since he was a competition winner, wasn't he? Still, I'm a heartless bastard).
Briers only redeems himself because he was so utterly brilliant in everything else that I've just got to believe that his performance in Paradise Towers wasn't his fault, somehow. Otherwise he'd be top in a heartbeat.
And I never much cared for Alexie Sayle's performance in the otherwise faultless Revelation, but that may well be because the character was so... SO bad.