Willis O'Brien's The Lost World is a wonder of an adventure movie, even 79 years later. The dinosaurs are primitive beyond belief, and you can see O'Brien is a prisoner of the limits of the stop-motion process, to the point that he doesn't have much leeway in choosing shot angles and lighting for his creatures (the visuals would be much better in the 1933 King Kong).
Still, the creatures have charm and stand out as characters--you see a Triceratops nuzzling its young after fending off a predator, you see an Allosaurus (what, no T-Rex?) looking at a loss after a Brontosaurus it intended to eat for dinner plummets from a cliff (it pauses at cliff's edge, as if not sure what it'll do next). Even better, since the movie's based on the Arthur Conan Doyle book (he makes a short appearance in the beginning), it's paleontologically accurate: the Brontosaurus is a passive plant eater, for one thing (unlike in King Kong, where he chased the explorers across the jungle). The human story is funny and and engaging and doesn't get too much in the way of the effects.
Great fun, though you have to realize what kind of effect this must have had on its audience then: people were so naive about cinema that they panicked when a train approached the camera and seemed about to crash into them. The Lost World must have seemed like a documentary, an actual window into another world.