If you wanna be literal, then it's the ioXtreme Prohttp://www.fusionio.com/products/ioxtreme/
But if you wanna be realistic, then.
Solid State Drives
After that I think it would be Ram-disks...
But then again some RAM SSDs are incredibly fast...http://www.storagesearch.com/ssd-fastest.html
Here browse that.
I guess it would partially depends on how you hook up the device to your system.
Excluding the PCI-E bus, cause that there is fast...