Startide Rising is the best of them all.
Sundiver is quite good too.
The later books were deeply marred by Brin’s giving into pressure from his editors to centre them on a group of adolescent males of diverse species because his publisher was of the view that the average scientific fiction reader was a 14 year old male. Brin has written about this and how difficult it was for him to write outside his natural quite adult style. His fantastic characters from Startide Rising are pushed into the background and only get to step forward and shine again at the very end.
So the childhood favourite ‘Mrs Frisby and the Rats of Nimh’ was onto something.