This reptile is the slowest.

Reptiles are air-breathing vertebrates covered in special skin made up of scales, bony plates, or a combination of both.

They include crocodiles, snakes, lizards, turtles, and tor- toises. All regularly shed the outer layer of their skin. Their metabolism depends on the temperature of their environment.

Unlike birds and mammals, reptiles do not maintain a constant internal body temperature. Without fur or feathers for insulation, they cannot stay warm on a cold day, and without sweat glands or the ability to pant, they cannot cool off on a hot one. Instead, they move into the sun or into the shade as needed. During cooler parts of the year they become inactive. Because of their slow metabolism and heat-seeking behavior, reptiles are cold-blooded.

Reptile reproduction also depends on temperature. Only boas and pythons give birth to live young. The other species lay their eggs in a simple nest, and leave. The young hatch days to months later. 

Giant tortoises don't usually move more than a mile and a half a day. While they may be slow on their feet, they have surprisingly quick minds and are smarter than most people imagine.