I think the show had a slow gradual decline from the time that JNT took over (last season of T Baker's run). That was where the obssession of the costumes began. But I wouldn't say the show ever really "sucked". It was still capable of gems, even in its last few seasons. They just became fewer and further between. The shorter season hurt as well. When you only had 14 weeks of Doctor Who, one bad serial could really disappoint. I'd watch the whole series, really, because people will disagree about which episodes are bad. Off the top of my head though, I would say the worst of the worst are the Gunfighters (Hartnell), the Dominators (Troughton, The Monster of Peladon (Pertwee), the Horns of Nimon and Underworld (Baker I), Twin Dilemma (Baker II; possibly worst serial of entire series), Timelash (Baker II) and The Happiness Patrol (McCoy; although it has its defenders).
You haven't watched the third Doctor yet? That's a great run.