The Secret of NIMH
Time Out says
Two years earlier, several Walt Disney animators (including Don Bluth) left the company to set up on their own, complaining that Disney was lowering its animation standards and heading for the dreadful cut-price techniques that are pumped out daily on children's television. This is their first animation feature, and visually they make their point well.
It's a spectacular return to the shimmering, mesmerising deep-focus animation associated with Disney's classic period: a marvellous use of lighting to create atmosphere, dew-drops glisten from every tree, and the villains are as primally terrifying as cartoon villains should be. The choice of material (Robert O'Brien's novel Mrs Frisby and the Rats of NIMH) is less fortunate, since it lacks the wonder of early Disney, and the mouse heroine is far too insipid and twee. It's still a pretty effective family film, though.