I think Four is my favourite Doctor after Nine and Ten (yes, they're tied for first place). He can be all crazy and random one second and then completely serious the next. Actually, this seems like an attribute all Doctors have, but Four did it in spades. Well, Ten can be just as crazy but he counters it with angst. ;-)
Part two of "Destiny of the Daleks" involves a lot of walking down corridors and "corridor acting". I love that it has a name; the sets were so short you had to make sure to get all your lines out before the corridor ended. I think it's in the commentary for "Planet of Evil" that Tom Baker said he came up with just stopping, saying a bunch of the lines, then starting off again as a way of accommodating.