Had you heard how Dory as a character came about in
FINDING NEMO? Screenwriter Andrew Stanton knew he needed a character with a memory problem, but how to make them non-pitiful but instead comedic? Then he heard Ellen Degeneres on TV in the background and her self-interrupting, stream-of-consciousness, happy-go-lucky persona was exactly the voice he needed for this character. He wrote her part rapidly after that, always knowing just what Dory would say.
So for
FINDING DORY, that same clarity of characterization and that same perfect voice (it was written for Ellen, after all!) are the two strengths of the movie. Another strength, most of the other voices and characters, especially Ed O'Neill as the grumpy grampa octopus with 7 legs. Not only is his voice perfect, the old guy has some of the greatest visual gags in the movie, with his ability to mimic and his single-minded drive toward his goal.
What's not the best? Originality. This movie has the same premise and many of the same plot tricks and character arcs as the first movie. My assumption? Everybody loved Dory so much in the first one they wanted her to be able to fix her main life problem. Now, you and I might think her main life problem is her memory glitches. But no. Like the first movie, her main problem is missing family members.
The first movie had lots of secondary characters whose job it was to reflect and expand upon the main premise in the movie, about parenting in a way that enables the children. This movie has only a few of those, and many more who are in there to jazz up the place. It's this repetitive plot combined with a less-brilliant screenplay that I think makes this movie come in a really clear second to the first one.
And yet! It's fun, it's emotional, it has wild shenanigans, it has Sigourney Weaver as herself! It's a good summer family movie.