Short answer: probably the dorsal root ganglion in the blue whale.
Initially I thought it would be a motor axon in the sciatic nerve, but after consideration I'm pretty sure that the dorsal root ganglion (DRG) has a longer axon than the motor information carried in the sciatic nerve (which is the longest nerve in the body, but not axon).
The DRG is a weird neuron because it's unipolar, so it's got a loooong axon, where one end has receptors in the skin and the other end enters the spinal cord, ascends in the fasciculus gracilis all the way up to the medulla in the brainstem, and synapses in the nucleus gracilis before then continuing to send information up to the thalamus and, finally the primary somatosensory cortex for "conscious" perception.
Note that for the sensations in the toes, this means that the axon goes all the way from the toe to the medulla, which is at about the same height as the mouth. This can be more than 2 meters long in tall people. That's a long axon!
Here's my logic for this answer:
- The DRG is the longest axon in the human body.
- Humans are mammals.
- The (confirmed) largest and longest animal to have ever lived is the blue whale.
- Blue whales are also mammals and thus have nervous systems roughly equivalent to humans.
- Therefore the longest axon in the blue whale, which is itself the longest animal, is probably the DRG.
When trying to confirm my answer, however, I learned a lot of crazy stuff. For example....
The largest blue whales are around 30 m long. This would suggest a DRG axon of at least 25 m, or 75 feet, long. Here's where it gets nuts and things stop making sense to me...
Axons typically conduct signals between a wide range of speeds: 0.5 to 100 m/s.
This means that if I were to flick a whale's tail (as one tends to do), it could take anywhere from a third of a second (a long time in brain time!) to more than SIX SECONDS to reach the whales' "conscious" perception (assuming they have consciousness).
Even more nuts, according to this paper:
...blue whale spinal axons growing at 3 cm/day represent an increase in volume that is likely more than double the volume of the entire neuron cell body—each day. This rapid volume increase for neurons is akin to the peak cellular growth rate observed for rapidly dividing cancerous cells.
Basically, these axons are growing faster than cancerous cells and the speed at which they stretch should cause them to tear or rupture.
Man, brains are crazy.
Smith, D. (2009). Stretch growth of integrated axon tracts: Extremes and exploitations Progress in Neurobiology, 89 (3), 231-239 DOI: 10.1016/j.pneurobio.2009.07.006