Depends on what you're looking for, really. "The City of Death", "The Deadly Assassin" and "The Talons of Weng-Chiang" are personal favorites of mine...all Fourth Doctor serials, of course. I also really, really like series 16, the one with Romana and The Doctor searching for the Key of Time. A few of the serials are snooze-fests (as many classic episodes tend to be), but the overarching plot is pretty interesting. Speaking of Romana, I also have a nostalgic fondness for Lala Ward as her second regeneration, though no particularly remarkable serials involving her immediately jump to mind. She's a really great actress, though, and tends to brighten the episodes she appears in.
Naturally, though, these are all Tom Baker serials, as he's my favorite Doctor (and the one I watched the most growing up). Other recommendations are "The Tomb of the Cybermen" (Second Doctor), both "Snakedance" and "Castrovalva" (Fifth Doctor), and the feature film, which while certainly not without some rather severe flaws, is mostly saved by Paul McGann's very strong performance as the Eighth Doctor. "The Five Doctors" is pretty entertaining as well, if as nothing else than mindless fanservice...it sort of suffers from not including Tom Baker, though. I haven't seen "The Three Doctors" yet, actually, but I've heard that it's similarly fun.
I sort of dislike the Third, Sixth and Seventh Doctors, so if you're interested in watching serials involving them you'll have to ask someone else.
