Needless to say,
The Stone of Cold Fire was by far my favorite sequel. XD
The Secret Of Saurus Rock comes as a close second. 6 and 7 were filled with very memorable characters, at least in my opinion.
Doc was mysterious, tough, and all-around cool! We'd never seen a longneck who could defeat a sharptooth (with the exception, of course, of Littlefoot's mother, and even Littlefoot and his friends on occasion, though it's done mostly by trickery or just being able to escape them, or even mere luck.) His vanishing into the distance toward the end was puzzling, too, and just added to his mystery.
I was actually thrilled when I first realized that the series included an episode with Doc's return.

And they did him pretty fair justice, too!