Q:

What's the fastest way to kill a server?

For the heck of it, what's the fastest way to kill a server, other than just pulling the plug? 
Kill as in a(n) (un)safe shutdown, you could theoretically reboot it and have it survive the incident. 
Edit: Tangentially related to serverfault.com, my apologies. IMHO, the question is related in that these are possible things users could be attempting, as well as in the case of adverse conditions there might be reasons server admins want to take down multiple boxes lest undefined "damage" might occur.
Edit2: opened to community wiki.

A:

give an oracle dba the root password.

A:

On a Sun server, Stop-A suspends the kernel immediately.  You can press the key combination or send an equivalent signal on the serial console.
On Linux, Alt-SysRq-O crashes the kernel immediately.  Through a remote shell, you can do echo o > /proc/sysrq-trigger.  There are other magic SysRq keys that will cause an immediate unclean reboot.  Alt-SysRq-S is handy to sync to disk first.

A:

C4, or other explosive

