Legendary sudo rm -rf /
In the interest of science, a VM died to answer this question:

testrm:~ admin$ sudo rm -rf /

WARNING: Improper use of the sudo command could lead to data loss
or the deletion of important system files. Please double-check your
typing when using sudo. Type "man sudo" for more information.

To proceed, enter your password, or type Ctrl-C to abort.

Password:
rm: /dev/fd/0: Operation not supported
rm: /dev/fd/1: Operation not supported
rm: /dev/fd/2: Operation not supported
rm: /dev/fd/3: Not a directory
rm: /dev/fd/4: Bad file descriptor
rm: /dev/fd: Operation not supported
rm: /dev: Resource busy
rm: /home: Resource busy
rm: /net: Resource busy
rm: /private/var/log: Directory not empty
rm: /private/var/run: Directory not empty
rm: /private/var: Directory not empty
rm: /private: Directory not empty
rm: /System/Library: Directory not empty
rm: /System: Directory not empty
rm: /: Is a directory
testrm:~ admin$ 


Interestingly, it succeeded. Back in the Slackware 2 days, I tried this on Linux and after it deleted the dynamically linked libraries rm was using, it failed.