This is of course a subjective question with many plausible answers.
A peculiarity of
cd is that it has to be implemented by the shell itself: it cannot be an external command (one that runs in a separate program).
cd changes the working directory of the shell process, and only the shell itself can do that.
This is a poor question because
cd isn't the only such command.
fg are in the same boat.
cd existed before the others, which is one way to make it unique.
Another plausible answer would be
dd, which has an option syntax that is entirely unlike any other command (it was borrowed from IBM).
Of course you can make arguments for many other commands, such as
rm (the only 2-letter command that is purely destructive),
su (the only two-letter command that absolutely has to be setuid root),
at (the only two-letter command that is typically setgid),
sh (it's the shell itself, which makes it different from the other commands which are special-purpose),
ps (the two-letter command that varies the most between unices), etc.