In Toy Story, Why is Buzz the only toy that does not believe he is a toy?
Is this a stage of being a toy or does Buzz have some sort of neurological dysfunction?
Because all the toys shown in Toy Story are all old toys mostly. So they all know the reality. But Buzz Lightyear is a new toy who doesn't know about reality. If you have seen Toy Story 2 then you will definitely remember that when new Buzz appears he is not under the impression that he is a toy. Even Zurg who comes later and tries to kill Buzz because he also doesn't believe that he is a toy rather thinks that Buzz is his enemy.
In short Buzz is not the only toy who doesn't know that he is a toy and mostly every new toy come to this phase that he feels he is a real person and not a toy.
If a toy is new, they may not yet know that they are toys. Maybe that is the reason buzz didn't believe he was a toy. That was the character / personality of Buzz, that was also responsible for his reluctance to believe. Its just an interpretation though!