Not so keen on the historical ones where they land somewhere and then just decide to go wandering off, this time separately. So I don't rate The Romans quite as highly as many do
In The Romans though they didn't just land and wander off. They had been there for a month or more before the main story takes place, and even then The Doctor and Vicki are only out for a walk to the city when he is mistaken for someone else, setting up his story. Barbara and Ian are then kidnapped, setting up theirs.
My vote goes to The Dalek Invasion Of Earth. It was one of the earliest stories I remember watching and I far prefer it to their first outing. One of the rare occassions where the 'sequel' is better.