It's often said that your favorite Doctor is often the first one you saw. It would explain why Tom Baker remains my favorite. I will say that David Tennant is a strong second, though. As others have said, Tennant was best with Catherine Tate's Donna, while I think Baker's time with Elisabeth Sladen (Sarah Jane) is the Gold Standard of Doctor/Companion storytelling.
Though the budget constraints makes the original series (Docs 1-7) a bit of a turn-off for many viewers, I like that era because they had to rely more on plot and characterization rather than "effects" and "spectacle". I think the same part of my brain that likes old-fashioned soap operas (where the characters spent enormous amounts of time talking about stuff while sitting at the kitchen table) likes original Who, because they were largely studio-bound and relying on the viewer to imagine what is now just CGI'd into existence onscreen. The older series, however, is much easier to pick up and begin viewing at any point, since very few of those seasons had the "umbrella plots" that link an entire season together.
I think seasons 1-4 of the new era can be pretty straight-forward viewing as well. It's not until Moffat took over that the plotlines became overly intricate, a bit too "see how clever I am" for the casual viewer to just pick up and follow. I still find most of the Matt Smith era to be incomprehensible, written toward certain fan bases rather than a broadly-written drama for the entire family, and you "need a flow chart to keep up" (to coin a phrase).
